Add utility methods to parser API

Add ``Parser.get_contest`` method to look up contests by text.

Add ``Parser.get_result_jurisdiction`` method to look up
sub-jurisdictions by name.

DRY-up creation of results for sub-jurisdictions.

Create a mixin classs, ``ResultAggregatorMixin`` for other classes that
have related results.  This removes repeated definitions of ``results``
and ``add_result`` methods.

Mix that class into ``ResultJurisdiction`` so that instances can have
associated results.

Update the constructor of ``Result`` to add the newly created instance
to its associated jurisdiction, if present, when a new ``Result`` is
instantiated.

Addresses https://github.com/openelections/clarify/issues/7
Merge branch '7-parser_api_enhancements'