ENH: add escape parameter to to_html()
Treating DataFrame content as plain text, rather than HTML markup, by escaping
everything (#2617) seems like the right default for `to_html()`, however, if a DataFrame contains HTML ([example](http://stackoverflow.com/questions/14627380/pandas-html-output-with-conditional-formatting)) or text already HTML escaped, it results in either unwanted escaping or double-escaping.

Changes in this PR:
- make HTML escaping programmable through a new `to_html()` parameter named
  `escape` (default True), allowing users to restore old `to_html()` behavior (<=0.10.0) by setting `escape=False`.
- add `&` to the list of HTML chars escaped, so strings that happen to contain
  HTML escape sequences or reserved entities, such as `"&lt;"`, are displayed
  properly.

Good addition, but I think there are actually 3 cases to be handled, and 
disagree with your choice of default behaviour.

Escaping <> by default is important to prevent XSS. your PR
keeps that and that's fine.

Running the following example in qtconsole/ipnb (or opening the HTML in a browser,I guess)

``` python
import pandas as pd
class AsHtml(object):
   def _repr_html_(self):
       return pd.DataFrame(["<b>str<ing1 &amp; &lt; &a</b>"]).to_html()
AsHtml()
```

currently produces

```
<b>str<ing1 & < &a</b>
```

and with your PR it produces:

```
<b>str<ing1 &amp; &lt; &a</b>
```

It seems to me that when a user has HTML in his frame and is using `to_html()`, he 
would usually want those html entities to display properly (as & not &amp; for example).

So there are 3 cases:
1) escape only <> but display html entities properly (the current default)
2) escape everything (your PR's suggested default behaviour)
3) don't escape anything (`escape=False` in your PR)

I think 1 should stay the default, and 2/3 should become possible by 
specifying the `escape` argument.
 Thanks for the feedback.  It's interesting because I've come to embrace black/white escaping rules (i.e., text is markup or not) based on experience with web frameworks.  Mixing `<` and `&amp;` in the same string and expecting `&amp;` to render as simply `&` when `escape=True` is not what I would expect, but I may be biased.

In web frameworks, the general practice is to explicitly identify markup and treat all other text as suspect.  Below are two examples of how you'd add markup to non-markup text with `markupsafe`, written by the author of flask and also used by flask.

```
>>> from markupsafe import Markup
>>> bold = Markup("<b>%s</b>")
>>> print bold % "str<ing1 & < &a"
<b>str&lt;ing1 &amp; &lt; &amp;a</b>
>>> print Markup("<b>") + "str<ing1 & < &a" + Markup("</b>")
<b>str&lt;ing1 &amp; &lt; &amp;a</b>
```

Furthermore, one must explicitly identify strings as Markup to prevent double-escaping.

```
>>> print bold % Markup("str&lt;ing1 &amp; &lt; &amp;a")
<b>str&lt;ing1 &amp; &lt; &amp;a</b>
>>> print bold % "str&lt;ing1 &amp; &lt; &amp;a"
<b>str&amp;lt;ing1 &amp;amp; &amp;lt; &amp;amp;a</b>
```

I'd be interested to hear your feedback on the following patterns and whether there is a better API for pandas users to safely markup DataFrame content.

```
import pandas as pd
from markupsafe import Markup
from IPython.core.display import display_html

bold = Markup("<b>%s</b>")
display_html(pd.DataFrame(["str<ing1 & < &a"]).to_html(), raw=True)
display_html(pd.DataFrame([bold % "str<ing1 & < &a"]).to_html(escape=False), raw=True)
display_html(pd.DataFrame([bold % Markup("str&lt;ing1 &amp; &lt; &amp;a")]).to_html(escape=False), raw=True)
```
 That's nice, but I'm not sure that addresses the problem I mentioned.
The case not covered is when we want tags to be escaped (for example
`<script src=http://evil.com/diabolical.js></script>`), but '&amp' to display 
as &. 

Let me put it this way: suppose a user of pandas is happy with the way 
things currently work. Suppose your PR is merged, what would the user 
have to do to keep things behaving as they are?

If you're aware of security issues that make merely <> escaping ineffective,
please speak up.

Also note that GH displays '&' when you write '&amp;' in a comment, and GH 
is quite a popular web application :)
 I'm going 180 on this.
http://wonko.com/post/html-escaping
https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet

In a nutshell, you can't tell what markup context the output might somehow end up embedded in 
and so many characters are potentially dangerous, including quotes, ampersands, 
percentage signs, braces and alarmingly, still others.

It seems an unlikely attack vector, but common experience shows that's bad reasoning.
So +1 for an escape smackdown, and thanks @gdraps for stirring this back up.
 Merge status?
 it's the right thing to do but i'm afraid it'd really inconvenience some users.
defaults bikeshedding?
dunno.
 For strict compatibility with 0.10.1, either a third escape mode can be added (e.g., 'compat' in addition to True/False), in which & is not escaped, or & escaping can be removed from this PR.  The main goal was to add an escape parm to give users the ability to restore 0.10.0 behavior.
 I'm comfortable with you merging as long as you put something in the What's New so we can point to it
 Rebased to master and added a few words to RELEASE.rst and v0.11.0.txt.  Let me know if I only need to update one or the other.
 merged, thanks!
 Failing in master on p3.3

```
======================================================================
FAIL: test_to_html_escaped (pandas.tests.test_format.TestDataFrameFormatting)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.3_with_system_site_packages/lib/python3.3/site-packages/pandas-0.11.0.dev_d749b91-py3.3-linux-x86_64.egg/pandas/tests/test_format.py", line 307, in test_to_html_escaped
    self.assertEqual(xp, rs)
AssertionError: '<table border="1" class="dataframe">\n  <thead>\n    <tr style="text-align: rig [truncated]... != '<table border="1" class="dataframe">\n  <thead>\n    <tr style="text-align: rig [truncated]...
Diff is 1049 characters long. Set self.maxDiff to None to see it.
----------------------------------------------------------------------
Ran 2960 tests in 122.311s
```
 Shucks, I can look into it later today.
 FYI These are failing in 3.2

```
======================================================================
ERROR: test_rplot1 (pandas.tests.test_rplot.TestRPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tests/test_rplot.py", line 242, in test_rplot1
    self.plot.render(self.fig)
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 881, in render
    adjust_subplots(fig, axes_grid, last_trellis, new_layers[-1])
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 759, in adjust_subplots
    label1 = "%s = %s" % (trellis.by[0], trellis.group_grid[index / trellis.cols][index % trellis.cols][0])
TypeError: list indices must be integers, not float
======================================================================
ERROR: test_rplot2 (pandas.tests.test_rplot.TestRPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tests/test_rplot.py", line 252, in test_rplot2
    self.plot.render(self.fig)
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 881, in render
    adjust_subplots(fig, axes_grid, last_trellis, new_layers[-1])
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 753, in adjust_subplots
    label1 = "%s = %s" % (trellis.by[1], trellis.group_grid[index / trellis.cols][index % trellis.cols])
TypeError: list indices must be integers, not float
======================================================================
ERROR: test_rplot3 (pandas.tests.test_rplot.TestRPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tests/test_rplot.py", line 262, in test_rplot3
    self.plot.render(self.fig)
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 881, in render
    adjust_subplots(fig, axes_grid, last_trellis, new_layers[-1])
  File "/home/travis/virtualenv/python3.2_with_system_site_packages/lib/python3.2/site-packages/pandas-0.11.0.dev_fc8a679-py3.2-linux-x86_64.egg/pandas/tools/rplot.py", line 756, in adjust_subplots
    label1 = "%s = %s" % (trellis.by[0], trellis.group_grid[index / trellis.cols][index % trellis.cols])
TypeError: list indices must be integers, not float
----------------------------------------------------------------------
Ran 3093 tests in 300.638s
```
 Ok, `test_to_html_escaped` failed due to unsafe reliance on dict key ordering.  The character escapes were stored in a dict, even though & must be escaped first to prevent double escaping.  py3.3's hash randomization was wonderfully effective at uncovering this every other run.  I pushed a new commit to this branch with the dict replaced by an ordered dict.  Let me know if I should open a new PR instead.
 cherry picked in master. Try and get travis-ci installed, it'll do the py3 testing for you
if you don't use tox.
