Fix #5 - Add checks for primary and autoincrement

 * Return provided value for primary key if given on insert
 * Check to ensure autoincrement is defined before returning
   'lastInsertId' or equivalent from `insert` method
 * Allow custom 'sequence_name' for PostgreSQL autoincrement
 * Validate primary key as required if primary but not autoincrement
Merge pull request #6 from vlucas/ticket_5

Fix #5 - Add checks for primary and autoincrement