Merge pull request #1 from ghing/master

Models and management commands to create Convictions from Dispositions
Custom QuerySet and Manager to query by crime category

Add custom QuerySet and Manager classes to query Convictions based
on crime categories.

These categories were suggested by Tracy and are based on a list
of IUCR codes provided by CPD.
Merge pull request #2 from ghing/master

Add census data
Merge branch 'master' into crime_categories
Refactor custom QuerySet and Manager classes

Move custom QuerySet and Manager classes.  Add some class methods
referencing other model classes to avoid circular imports.

Use django-model-utils' PassThroughManager for the manager of
the Convictions model to avoid boilerplate proxying methods to
the custom QuerySet.
Don't cascade when deleting convictions
Merge remote-tracking branch 'sc3/master'
Merge branch 'crime_categories'