Add Python API docs

After helping orient some folks during the National Day of Civic Hacking, I realized that we really needed Python API documentation, particularly for the datasource modules.  In the past, we had been directing people to particular state's implementations, but I found this to be problematic because it's difficult for me to remember what's going on in a particular state, and each state's implementation is varied enough that a user would have problems knowing what to take note of and what to ignore.

Add documentation, based on the class docstrings, for the
BaseDatasource class to give contributors a better idea of which
methods they need to implement in their state subclasses and
the expected return values.

Also provide an example datasource module implementation for
the simplest possible case.

Addresses https://github.com/openelections/docs/issues/5