Convert Java::JavaUtil::HashMap to ruby Hash in jdbc adapter, for better handling of PostgreSQL hstore type (Fixes #686)

In general, the jdbc adapter should convert java types to ruby
types.  This wasn't being done for the hstore type, so Java
HashMap instances were being returned.  The typecasting code
doesn't know what to do with such instances, so it wasn't
converting them to Postgres::HStore instances.

With this patch and the typecast_on_load plugin in the model,
model instances will have hstore values returned as
Postgres::HStore instances.
Mention typecast_on_load plugin in pg_typecast_on_load RDoc