Added some missing type mappings and aliases for existing ones.

Using [PostgreSQL sample database](http://www.postgresqltutorial.com/postgresql-sample-database/) some fields were not generated. In particular case, the `smallint` type was missing. Added this and some other more common types to mapping.

Couldn't get the explicit join I wanted to try out working though, because it appears, that the schema is defined in a way that different types are used at the either side of foreign key constraint (`smallint` vs. `integer`):

![Matching types](http://i.imgur.com/rUIX3ab.png)

Don't know whether to consider this as feature for finding inconsistencies in database schema or not :)