Minor documentation updates

Great library, thanks!  

A few minor documentation typo fixes.  When trying to run the tests, I noticed that mock wasn't being installed automatically due to the order of operators in ``setup.py``.  Finally, adds a gitignore file (from https://github.com/github/gitignore/blob/master/Python.gitignore) for convenience.

As a side note, I noticed that much of the content is the same between the readme and the guide in the docs.  It's possible to use the rst directive ``.. include :: ../README.rst`` inside ``docs/guide.rst`` to automatically bring in content from the readme.  You'll likely have to play around with the headings to get things right in the Sphinx output, but that might help reduce the double-up.