Separated adapters from model #581
Memoization of sorcery_adaptor
Minor
Fixed DM current_users method
Adapter capability and proper calls to adapter.find
Use adapter find method instead of ORM's

DataMapper `find` method returns enumerator. To return a single record we need
to use our own `find_by_id` method which always returns one result
Fixed missing methods in adapters
Merge pull request #590 from NoamB/separated-adapters

Separated adapters from model #581