Support NULL values

According to the PostgreSQL [documentation on hstores](http://www.postgresql.org/docs/9.0/static/hstore.html) keys and values are just text:

> Keys and values are simply text strings.

In djorm-ext-hstore `HStoreField.get_prep_value()` takes care of that and converts everything non-string-like to a string. However, documentation continues...

> A value (but not a key) can be an SQL NULL.

So imho `None` values should not be converted to strings.