2.6+ fixes, replace :admin with :tags

Changes for RabbitMQ 2.6+. The primary change is dropping :admin from the user type in favor of  supporting arbitrary:tags. This also fixes the user type being broken for users that contain multiple tags in some cases due to regex issues and the other tags being blown away in other cases if :admin was true. It's not compatible with versions of RabbitMQ older than 2.6.0. The tests are updated and passing (hopefully properly I'm new to this) but it could use more :tags testing.