Add Python API docs

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
Merge pull request #15 from ghing/python_api_docs

Add Python API docs