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
Suggest using `Assignment.connection` instead of `ActiveRecord::Base` to support multiple database apps.