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.
Do you believe this should be checked if a string?