make oid/rowid functionality optional for Postgres
postgres has disabled oids by default as of 8.1 (rowids in sqlalchemy). make all useage of rowids optional, and have postgres by default not use rowids at all. SERIAL columns should use the default sequence <table>_<column>_seq to generate primary keys.
we probably want to make a special sequence internal to postgres that doesnt conflict with an existing "optional" sequence and modify sqlengine's _process_defaults() to give more control to the subclass in running a column's default generators.