Restores backwards compatibility with Django < 1.7
Python 3 compatibility fix
Restricted usage of custom lookups to instances of HStoreField
Additional tests for invalid values in custom lookups
Raise ValueError in WhereNode.make_atom if an empty list is passed as a parameter for contains and icontains lookups
Using `HstoreField.register_lookup` instead of `get_lookup` for custom 
lookups.
Removed useless db_type check in HStoreContains