Update docs which are currently wrong.
Added some cleverness to update()

It is now possible to call Model.update() either as an instance or a static method.

Instance version assumes the update query is {_id: self['_id']}:
    thing.update(<update)

Static version allows you to provide the query (essentially forwarding to the collection):
    Thing.update(<query>, <update>)
Updated docs.
Merge pull request #12 from tleach/tl-update

Implemented a class method variant of Model.update()