group: make 'result-as-the-first-parameter' from EventEmitters available

From http://howtonode.org/control-flow-part-ii:
> For cases where there are more than two events
> and/or they can be called multiple times, then
> you need the more *powerful* and *flexible* EventEmitters.

Using `group()` for `http.get()` which is EventEmitter isn't
possible with Step's only "The Node.js Callback style"...