Add County spatial data

Add a County model to hold County data from the TIGER shapefile.

Added an ``in_cook_county`` flag to the ``CensusPlace`` model.

Add a ``flag_cook_county_places`` management command to set the
``is_cook_county`` flag on CensusPlaces that intersect Cook County.

Renamed ``export_chicago_geojson`` management command to
``export_border_geojson``.   Updated the command to load the Chicago
boundary from the ``CensusPlace`` model instead of trying to aggregate
it from the border of the community areas.  Updated the command to add
both Cook County and Chicago's boundaries in the GeoJSON.

Update README to reflect all of this.

Addresses https://github.com/sc3/cook-convictions/issues/133
Better identification of places in Cook County

The updated spatial queries eliminate places that touch but
don't overlap the border of Cook County.

Addresses https://github.com/sc3/cook-convictions/issues/133
Better Chicago and Cook County borders

Rename the ``chicago_geojson_from_shp`` management command
``border_geojson_from_shp`` and update it to take a second
argument for a Cook County shapefile.

Update the command to use the cartographic shapefiles as they
remove the offshore areas of Chicago and Cook County.

Update the README to reflect the new command name, the additional
argument and the new datasets.

Addresses https://github.com/sc3/cook-convictions/issues/133
Merge branch 'master' into better_iucr_code_detection
Update loading of final statute and IUCR fields

Add final_statute_formatted field to the Disposition and Conviction
model.

Add a format_statute() function to format the parsed statute that will
go into final_statute_formatted.

Factor final_statute field population out into a method,
load_final_statute.

Remove trailing whitespace from files edited in this commit.

Rename statute2iucr management command to load_final_statute_and_iucr to
reflect these changes.

Fix statute.strip_surrounding_parens to handle empty string

Addresses https://github.com/sc3/cook-convictions-data/issues/7,
https://github.com/sc3/cook-convictions/issues/83