Collection#create returns result from Model#save

It feels natural that all asynchronous methods return an xhr. `create` is, as far as I can see, the only one that does not. I can see the value in it returning a model, as it might be created from an object within `create`, but I feel returning an xhr leads to a better API. It's also more consistent with the other async methods. If the model is needed directly after `create` is called instead of when the event is triggered, I think it's better to create it yourself, call save on it and add it to the collection.

I have not fixed the failing tests as I thought I'd first see what you guys think. (I didn't find any earlier discussion on it, but that might just be my limited searching skills.)

Any thoughts?