[contib.postgres] HStorefield

Beginnings of an `hstore` implementation. TODO:

- [x] support migrations to create the extension and register with psycopg2
- [x] expand the field definition, including restriction of variables
- [x] form field
- [x] add lookups
- [x] docs
- [x] validator

There is a design decision to make: do we want a simplistic hstore field which follows the rules of hstore itself - restricts to strings and null, or do we want something a bit more clever like `django-hstore` provides that does some json encoding on non-string types? I currently lean towards the former as we will also have support for a `jsonb` field for more rich data. This would also remove a lot of complexity from the `django-hstore` implementation - descriptors, subclasses of dict etc.
Yes please. Updating the examples to show the difference would also be helpful. I'm not sure it's really possible to tell the difference from the dog examples so far.