Modified postgres adapter to support insert ignore policy m...

...achine assignments

569 examples, 0 failures
Coverage report generated for RSpec to /Users/yzhang/the_policy_machine/coverage. 629 / 707 LOC (88.97%) covered.
Passes with postgres and mysql adapters and in Dalton
`Assignment.find` seems like an unnecessary trip to the database. `Assignment.new(parent_id: parent_id, child_id: child_id).add_to_transitive_closure if result.count == 1` would probably work the same way with one fewer DB call.