Make Model().update delegate to the Python dict API 

review @tleach 

Make Model().update delegate to the Python dict API.
Currently, Model().update overrides the dict API in favor of proxying Model.collection.update. This was nice, but ultimately the cost of not being able to tread Models like dicts was too high. We will now have the
following methods:
    
-    Model.update            (proxies Model.collection.update)
-    Model().update          (proxies dict.update)
-    Model().update_instance (proxies Model.collection.update)