Add suburbs to analysis

Add a CensusPlace model to represent census places in Illinois.
I did this iteratively, so there are a couple of migrations
reflecting these schema changes.

The 0017 migration didn't create the spatial fields or indexes
for some reason, even though the migration code seemed correct. I
had to create the spatial column and indices by hand, running the
queries reported by:

./manage.py sqlall convictions_data

In our analysis, we're only concerned with suburban Census Places,
so I added a management command, ``flag_chicago_msa_places`` to
set a flag on the model from records in a CSV file,
``data/data/tl_2010_17_place10_chicago_msa.csv``.

I updated the ``geocode_dispositions`` management command to
support geocoding non-Chicago records.

I updated the ``boundarize`` management command and model methods
to also try to detect an address being in a Census Place.

Add a management command for setting conviction places from their
related disposition.

Rename the ``export_ca_geojson`` to be ``export_model_geojson``
and update it's functionality so it can also export GeoJSON for
the census places around Chicago.  Also update the underlying
model, manager and queryset methods to support this.

Add a ``per_capita_convictions()`` aggregate method to the
``CensusPlace`` and ``CommunityArea`` managers and querysets.

Addresses https://github.com/sc3/cook-convictions/issues/77