Moves update code into a separate module.

@corydobson @parkr
I have plans to implement the  [findAndModify](http://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/) command. `findAndModify` will share the update code with `update`, so it needs to be factored into a separate method. To keep `processor.js` simple, I think the update code also merits to be in a separate module.
It will probably end up being used from more than one module, and at that time, I'll move it. Right now the helpers module is mostly about functionality deserializing network packets. I would like to split it into the deserialization module and the remainder which will be a grab bag of functions.