Add psycopg2 type overrides to DBAPI

The SQL Layer is just treating "SELECT '2013-01-01';" as SELECTing a string, since there's no type information. When the value is returned, the OID won't correspond to a datetime OID. Actually test_psycopg2 already included a test for datetime using "SELECT CAST('2013-10-5' AS date)" and confirming that a date is returned.