MongoID 3.0 compatibility + refactoring

Hey Michael,

I've added a patch for mongoid 3.0 compatibility + refactoring things a bit, moved the `include Paperclip` calls into the `self.include` section where it belongs to.

Also, if you don't mind i've made the original `has_attached_file` method work, it seems a bit silly to define your own method in this case.

Cheers,
Nik