Rails 4.2

All tests green with Rails 4.2.0-beta2 and a nasty Arel fix that plagues Rails 4.2 testers. **Not** tested within a real application though.

Rails 4 deprecated and Rails 4.2 enforces the use of `find_or_initialize_by` only with a hash parameter, so that was the only change made.

I think this change makes it compatible with Rails 4 in general (not fully tested). It certainly makes it *incompatible* with Rails 3.2 though, that might need another round of fixing. What are your thoughts about Rails 4 compatibility?