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)
bump Mongothon to 0.6.0 (backward-incompatible API change)
fix README formatting
Merge pull request #19 from gamechanger/by-model-update

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