Add Sphinx documentation

This pull request adds the minimal configuration needed to build HTML documentation using Sphinx.

Voluptuous works out-of-the-box for Sphinx (one req. change in cb2e997). The only caveat is in order to reuse the content of the README file within the docs, the README must be in rST instead of Markdown (534f335).

The end result [looks like this](http://whiteinge.github.io/voluptuous/index.html). Noteworthy additions we get for free are the [index](http://whiteinge.github.io/voluptuous/genindex.html) and the [module API reference](http://whiteinge.github.io/voluptuous/reference.html). Getting automatic builds via [ReadTheDocs](https://readthedocs.org/) is as simple as checking a box in the GitHub "Services" settings for the project (plus they use a nicer HTML theme).

If you'd prefer not to buy-in to the rST/Sphinx ecosystem feel free to just close this pull req. I will not be offended. If you are interested, I'm available to help with questions/fixes/additions going forward in the future.