Using Sphinx to generate API docs for PySpark.
requirement: Sphinx
```
$ cd python/docs/
$ make html
```
The generated API docs will be located at python/docs/_build/html/index.html
It can co-exists with those generated by Epydoc.
This is the first working version, after merging in, then we can continue to improve it and replace the epydoc finally.
Author: Davies Liu <davies.liu@gmail.com>
Closes#2292 from davies/sphinx and squashes the following commits:
425a3b1 [Davies Liu] cleanup
1573298 [Davies Liu] move docs to python/docs/
5fe3903 [Davies Liu] Merge branch 'master' into sphinx
9468ab0 [Davies Liu] fix makefile
b408f38 [Davies Liu] address all comments
e2ccb1b [Davies Liu] update name and version
9081ead [Davies Liu] generate PySpark API docs using Sphinx
