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.
Handle multiple values from get_iucr()

get_iucr() returns a list.  Set the appropriate model fields
only if there is only one matching IUCR offense for the statute.

Addresses https://github.com/sc3/cook-convictions-data/issues/5
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'
Fix typos in queries

Generally, s/iucr__in/iucr_code__in/. Embarassing.
Add better queries for manufacture/delivery of drugs

These queries are based on looking through the charge descriptions
and statutes of many records and building queries that take both
into account.

Factor drug queries out into a  separate module.

Use Sphinx to create the skeleton of some docs.

Addresses #9
Tweak manufacture and delivery drug charge queries

Fix some typos, mostly

Addresses #9
Reorder class X queries

Move class X queries to the top so the queries follow the
alphanumeric ordering in the ILCS.

Remove trailing whitespace.

Addresses #9
Add queries for drug possession

Addresses #9
Add changes to IPython notebook

Add drug-related inquiries.

Addresses #9
Merge pull request #8 from ghing/master

Merge in some queries, mostly related to drug charges