Oauth2 server php couchbase

This pull request would provide a storage class for Couchbase Server. An important addition considering Mongo is supported currently out of the box and Couchbase is one of it's largest competitors in the space.

Passwords are by default stored in plain text as per the mongo implementation and an expectation to override the relevant methods is made to provide a secure implementation.
I imagine we don't want a `MongoConnectionException` here.  Is there a `Couchbase` equivalent?