Add support for the :class_name option in belongs_to

Change-Id: I6aa6e6f6c23e761ccad72edfb45788c81a1bea49
Reviewed-on: http://review.couchbase.org/29821
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>