Add Couchbase ORM

Hi @tute we've created a [Couchbase](http://www.couchbase.com/) ORM for doorkeeper.

I just noticed another pull request where you said you were planning on removing ORMs completely from doorkeeper? applicake/doorkeeper#281

Does this mean that ActiveRecord will be removed too? As you can see below we had to comment out quite a bit of ActiveRecord specific code and we were going to ask how you would like us to manage those changes.

Are there any guides for building ORMs for the new version of Doorkeeper?