Support CouchDB _updates

I think it would be very useful to support the use of CouchDB update handlers.

Comments in the code:

```
# jquery.couch.js uses the same method for updating as it uses for creating a document, so we can use the `create` method here. ###
  update : (model, opts) ->
    @create(model, opts)
```

but jquery.couch.js has db.updateDoc. It's purpose is to execute update handlers.
