Load PollingLocations from VoterInfo Query for Test Election (2000)

This PR does the following:

Reorganizes header files for each source file in the following way, with all includes in the .h files:
  * Apple includes
  * 3rd party lib includes
  * project includes

Cleans up the database schema so that most attributes are optional, since data is so sparse.

If a voterInfo query is made and successful, all objects related to the election instance are cascade deleted and recreated from the new response. This is done because it is not guaranteed that a voterinfo query returns an id for each object in the response, so we cannot guarantee that we will be able to reference that object sometime in the future by an id.

Unit tests for the added functions on the models.

Remaining to implement:
  * State (and child references)
  * Contest (and child references)