remove acts_as_messageable monkeypatch to activerecord
it is just as easy to "include MessageCenter::Models::Messageable" in the model and not need to extend active_record
remove methods from Messageable for manipulating Items, Conversations, and Receipts
all of the methods for managing items can be called on the items themselves.
putting all of these methods on Messageable means *many* methods get added
to the application User class and have a good chance of conflicting.
this change is meant to reduce the overall API and limit the impact to User