Mongoid 4

Mongoid 4 was released!
I know the support for this version was planned for 1.x, but I thought we could do some smaller release, which could support the new Mongoid.

One more change I need to make is to solve https://github.com/NoamB/sorcery/issues/405 - I'll do it later this week, but I'm not sure yet if I can do it without breaking existing sessions (i.e. logging all users out)