Add Documentation

[HHammond]

Addresses https://github.com/HHammond/PrettyPandas/issues/12

This PR will add documentation to the project. This will be done using ReadTheDocs and Sphinx.

TODO:

• [x] Rebase from master
• [X] Setup ReadTheDocs.org hooks
• [X] Setup Sphinx
• [x] Write documentation
  • [x] PrettyPandas API
  • [x] apply_pretty_globals
  • [x] PrettyPandas
    • [x] Constructor
    • [x] total
    • [x] average
    • [x] min
    • [x] max
    • [x] median
    • [x] summary
    • [x] multi_summary
    • [x] as_money
    • [x] as_percent
    • [x] as_unit