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
Export Community Area GeoJSON

Add a management command and some custom manager and queryset
methods for exporting Community Area GeoJSON.

Adds a dependency on django-geojson, though we only use their
serializer class.

Addresses https://github.com/sc3/cook-convictions/issues/48
Add additional final fields to Disposition, Conviction

Add the following fields to the Disposition and Conviction models:

* final_chrgdesc
* final_chrgtype
* final_chrgclass

These make Conviction queries easier, especially because statute ids
can be hard to interpret and might not map to IUCR codes.

The fields trickle up from RawDisposition and can be loaded with this
snippet:

from django.db import transaction
from convictions_data.models import Disposition

with transaction.atomic():
    for d in Disposition.objects.select_related('raw_disposition').all():
        d.load_field_from_raw('chrgdesc')
        d.load_field_from_raw('chrgtype')
        d.load_field_from_raw('chrgclass')
        d.load_field_from_raw('ammndchrgdescr')
        d.load_field_from_raw('ammndchrgtype')
        d.load_field_from_raw('ammndchrgclass')
        d.save()
Merge pull request #2 from ghing/master

Add census data