Add SMALLSERIAL type support on PostgreSQL 9.2 platform

Since PostgreSQL 9.2 there also is autoincrement support for `SMALLINT` type columns. The type to use is called `SMALLSERIAL`. This PR adds support for smallint autoincrement columns on `PostgreSQL92Platform`.
Please not that no further type mapping is necessarry, as `SMALLSERIAL` columns internally map to smallint in PostgreSQL which is already covered by `PostgreSQLPlatform`.