Add support to Mongoid 4.0 (edge version)

Hello,

I am actually using Doorkeeper on a brand new Rails 4.0 application. Because of that version of Rails I was forced to upgrade Mongoid to its current version (4.0, [master](https://github.com/mongoid/mongoid) branch). However, at the moment, Doorkeeper only supports _Mongoid  3_ and can't handle the edge version, mainly because of a namespace change. Indeed,  ```Moped``` namespace is no longer required, then instead of using ```Moped::BSON::ObjectId``` we should directly use ```BSON::ObjectId```.

For the convenience I added a ```:mongoid4``` orm option and updated generators and specs.

I'm not sure if there's a way to make exclusion rules in the Travis build matrix less verbose. So atm, I followed your current pattern.

Specs are green.

Let me know if I missed something or if I can improve some of my modifications.

Thank you.

edit:  Ok, I've just noticed that you are actually planning on removing ORM specific code out of doorkeeper.