Added Stata 13 support
Added initial Stata 13 .dta file format support. Newly added string features ("strls") and writing are not supported. Fixes #4291.

@PKEuS you have travis turned on?
 Yes.

Main error shown on travis is "ValueError: Version of given Stata file is not 104, 105, 108, 113 (Stata 8/9), 114 (Stata 10/11), 115 (Stata 12) or 117 (Stata 13)" for all 117 unit tests, but only for Python != 2.7. Does anybody have an idea what is going on?
 the one build that passes runs very few tests (it's for a non-English testing) and doesn't run the stata stuff

does this pass locally?

Travis runs on 32-bit Linux FYI 
 Yes, it passes locally. But there is something strange about it I noticed:
When I execute the tests a second time, they fail with the same message as Travis, while they pass, after I changed something in stata.py.
 maybe sure that the multiprocess_can_split is False (at the top of the test_stata)

what did u change?
 I ran this PR local (64-bit linux) and got this; maybe reading the wrong value for vartypes?

```
(Pdb) p self.path_or_buf
<open file '/mnt/home/jreback/pandas/pandas/io/tests/data/stata1_v13.dta', mode 'rb' at 0x2d5d150>
(Pdb) p seek_vartypes
1441433355735269392
(Pdb) self.path_or_buf.seek(seek_vartypes)
*** IOError: [Errno 22] Invalid argument
(Pdb) p self.byteorder
'>'
```
 I was able to fix some problems, now two of 5 buildbots (2.6 and 2.7) are still failing. Now with:

> File "/home/travis/virtualenv/python2.6_with_system_site_packages/lib/python2.6/site-packages/pandas-0.0.0-| py2.6-linux-x86_64.egg/pandas/io/stata.py", line 362, in _read_header
>     self.path_or_buf.seek(seek_vartypes)

Might the problem be that seek_vartypes is read as an 8-bytes integer?
 you should be explicit if you are reading bytes
Travis is a 32 bit build
 > seek_vartypes = struct.unpack(self.byteorder + 'q', self.path_or_buf.read(8))[0] + 16

Is that not explicit enough? Where do I have to add something? At the line above, or the following one?

> self.path_or_buf.seek(seek_vartypes)
 @PKEuS not sure. can you setup a vagrant 32-bit machine and debug?
 Finally, the problem seems to be solved!
Endianess was detected wrongly - no idea why it worked with python 3 - apparently python 2 was correct. My code was simply wrong.
 @PKEuS great, couple of questions

it looks like this is backward compat (as you can still read the new files). Is there a change in the writing behavior?

can you document in doc/source/release.rst and update the docs in io.rst (I would just add a short blurb that in v0.13 you now accept this new Stata format, while remaining backwards compat)
 The StataWriter has not been changed - it still uses the old format.

I will add the new feature to the release notes.
 @PKEuS this looks fine

can you rebase on master, then squish down to a few commits....?
 Squashed to two commits and rebased.
 @PKEuS looks fine to me ... @jseabold ?
 I defer to the test suite ;)
 @PKEuS thanks again!
