[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.
This is the way lookups (and SQL in general throughout the ORM) is written currently. We could pick some other way (and the latter one is clearly more readable), but it is best to keep this file consistent with the rest of the code base.