API change. Removed managers for Attributes and Tags - these are instead incoorporated into the TypedObjectManager which all Typeclassed object handlers inherit from. This concludes the refactor started in #529. This means that access to Attributes/Permissions/Aliases and Tags/nicks are done directly through e.g. ObjectDB.objects.get_by_tag() (or ev.managers.objects.get_by_tag) rather than using Tag.get_obj_with_tag and specifying the model type manually. The create_tag() method was also moved into the TypedObjectManager as part of removing the Tagmanager. As part of this change, Tag and Attribute was also removed from the ev interface.