Handle multiple values from get_iucr(), custom querysets for crime categories

This includes the fix for #5 as well as the custom QuerySets and managers for querying convictions by crime categories.

It factors the custom queryset and manager classes out of convictions_data.models and into convictons_data.query and convictions_data.manager.

Finally, it fixes the delete cascade handling on foreignkey fields when deleting Conviction model instances.