Feature/rails 3.1 compatibility

This is a feature branch that gives us rails 3.1 compatibility. It consists mostly of a mixin that tests the rails version so that we only call set_primary_key in rails 3.1 applications as it's deprecated in 3.2. Also manually specifies the id in the new clip form so that the js is the same for the preview ajax load.

@lightswitch05 please review.