Possible fix for #44: wrong prepare method

I have removed `prepare(self, connection)` method because it has wrong signature for django (see: https://github.com/django/django/blob/stable/1.6.x/django/db/models/fields/__init__.py#L357)
Django call prepare method of values without connection.

This can be a possible fix for #44 
Also, the connection is not used in HStoreDict for nothing (As far as I know).