Create Postgis9Dialect, Postgis92Dialect and Postgis94Dialect

PostgisDialect extends PostgreSQL82Dialect with Postgis specific datatypes but PostgreSQL82Dialect has been extended with PostgreSQL9Dialect which is extended by PostgreSQL92Dialect extended by PostgreSQL94Dialect.

These PostgreSQL9*Dialect classes adds features from newer Postgresql versions.

At the moment one have to choose between using PostgisDialect to get Postgis features and a PostgreSQL9*Dialect to get access to the latest Postgresql features.

I suggest to create a PostgisDialect per PostgreSQL9*Dialect to get both sets of added features.