AssocationSet::build

I've been wanting to do this for a long time. Do you think it's a fit, @pushrax?

You can do this now:

```coffeescript
newChild = parent.get('children').build(someProperty: "some value")
```

No docs yet -- I want to give associations proper docs soon though. Also, is there a better way to get the parent record from the assocationSet or was I right to implement that here?