Add methods to create, read and delete Authorizables

clobber_unauthorized will take a user, determine which fields
they have read privileges for, and set the rest equal to none.
Fixes issue #20
Merge pull request #23 from ahinz/fix/tinybug

Fixes issue #20
Add documentation for mixins
Split up tests
Add static method to clobber entire querysets
Merge pull request #22 from steventlamb/topic/add_role_models

Add methods to create, read and delete Authorizables