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 treat Models like dicts was too high. This would change `Model`s to have the
following methods:
    
-    `Model.update`            (proxies `Model.collection.update`)
-    `Model().update`         (proxies `dict.update`)
-    `Model().update_instance` (proxies `Model.collection.update`)