CLN: added io.api for i/o importing functions
closes most of #3411 (all except the deprecation of `from_csv`)
- moved excel functionaility out of io.parsers to io.excel
  added read_excel top-level function
  aliases from pandas.io.excel
- added read_stata top-level function, to_stata DataFrame method
  aliases from pandas.io.stata
  removed read_dta (replace by read_stata)
- added read_sql   top-level function, to_sql DataFrame method
  aliases from pandas.io.sql

DOC: doc updates for all the above and intro section to io.rst

@wesm pls give this a once over
 @y-p @wesm ok on this?
 IIRC, users rely on ExcelWriter directly to write mutliple tabs to the same file, 
these changes would break (at least) that, no?
 nope...its still there (and still exported by default)
 I must be missing something, how is this not a breaking change?

``` python
In [1]: from pandas.io.parsers import ExcelWriter

In [2]: from pandas.io.parsers import ExcelFile

In [3]: pd.__version__
Out[3]: '1.11.0.dev-6d08f1e'
```

(Note, I just pushed a change to master turning 0.12.0->0.11.1)

This PR:

``` python
In [1]: from pandas.io.parsers import ExcelFile
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-a9154a289dc3> in <module>()
----> 1 from pandas.io.parsers import ExcelFile

ImportError: cannot import name ExcelFile

In [2]: from pandas.io.parsers import ExcelWriter
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-2-39a586c0fd73> in <module>()
----> 1 from pandas.io.parsers import ExcelWriter

ImportError: cannot import name ExcelWriter

In [3]: pd.__version__
Out[3]: '0.12.0.dev-f7ad3c3'
```
 yes, that is an API change, let me see if I can do a warning on that, or let it break?

in any event, the top-level namespace

```
import pandas as pd
pd.ExcelWriter
```

succeeds
 I updated to give a warning if you try to use an `ExcelWriter/ExcelObject` imported from pandas.io.parsers

do you know if its possible to do this on import though?

e.g.

```
from pandas.io.parsers import ExcelWriter
ExcelWriter('foo.xls')
```

the warning won't print till the `ExcelWriter` line; any way to actually have it warn on that particular import?
some kind of import hook?
 Scratch that, Obviously everyone will get the object from the toplevel namespace. 
 actually, I think you have a valid point (as the docs _did_ originally have you import from `pandas.io.parsers`)
 If you think so, then this doesn't belong in a minor release. 
The semver contract is "if you upgrade a minor release, your code won't break", not "you'll get a warning".

Other then that, a welcome cleanup of an increasingly crowded space.
 ahh...but I have made that so, you WILL get a warning (and your code wont' break)

ok....will proceed then...
 @wesm any comments?
