Hstore delegate

All seems in order except there are two test that fail:

[First](https://github.com/igorkramaric/daffodil/blob/hstore-delegate/test/test.py#L200) one fails because of nature of POSTGRES - if there is an unknown field in query - no records are returned.
No idea what could potentially be a workaround. Probably there isn't one - just overriding this test with pass?

[Second](https://github.com/igorkramaric/daffodil/blob/hstore-delegate/test/test.py#L29) one is All Records ({}). I'm not able to distinguish that from "[]". Even [the solution](https://github.com/igorkramaric/daffodil/blob/hstore-delegate/daffodil/django_hstore.py#L12) for "[]" is not too nice.

I keep model in separate folder, hope it isn't wrong. This way it's aligned with how JSON data is stored (in separate folder too).