Revamp pymssql documentation

To do list
----------

- [x] Port relevant documents from Google Code wiki to Sphinx-flavored reStructuredText (reST).
- [x] Change Sphinx theme to use the readthedocs.org one.
- [x] Initial cleanup.
- [x] Add metadata markup to get cross-linking.
- [x] Add a features list to the front page.
- [x] Make it clear pymssql only supports FreeTDS now, even on Windows.
- [x] Add Cython to dependencies.
- [x] Make sure we recommend FreeTDS >= 0.91.
- [x] Unify reference to pymssql 1.0.x and refer users of such version to the Google Code wiki. Remove all the other mentions.
- [x] Research if we can include the reST-formatted files located in the project root (`README.rst`, `README_building_and_developing.rst`, `Changelog_highlights.rst`, `TODO`).
- [x] Add Marc Abramowitz to the credits for 2.0.
- [x] Publish on Read The Docs (http://readthedocs.org) (RTD).
- [x] Integrate GitHub and RTD for automatic docs rebuilding.
- [x] Host the docs at `pymssql.org`.
- [x] Remember to change `setup.py` links to point to `pymssql.org`.
- [ ] Add analytics?
- [x] Add reference for DB-API 2-mandated methods, attributes (`pymssql.connect()`, `pymssql.apilevel`, ...).
- [ ] Compare the documented public symbols with the ones actually published by the Python modules by browsing the `.pyx` files.
- [ ] Try to enhance the building and using on Windows experiences by reusing/adding documentation.
- [ ] Expand building instructions to take in account current Python practices (virtualenv/pip)?
- [ ] Try adding a "Fork-me on GitHub" ribbon.