Add Hstore support

Adds support for Rails 4 Postgres Hstores.

With Hstores, calling `#{name}_before_type_cast` always returns an empty string (I haven't dug in enough to find out why), but calling `name` directly returns a hash, which is then transformed into an insert statement correctly in the `values_sql_for_columns_and_attributes` method.

If the user isn't using Postgres or there are no Hstore columns, they won't even notice the difference.

I wasn't able to get the tests running (the MySQL gem wouldn't install and I don't really care to work out why) but I have this running successfully in my project. That said it is probably worth adding tests if anyone else is able to do so.