Add model for census tract

Addresses https://github.com/sc3/cook-convictions/issues/46
Add census data

Add fields to the CensusTract and CommunityArea models to hold
total population and per-capita income from the ACS.

Add a management command to load census data from an American Fact
Finder CSV file.

Add manager and model methods to aggregate census data from tracts
to community areas.

Addresses https://github.com/sc3/cook-convictions/issues/46