Emitter(obj) to extend a proto

[tj]

Probably too sugary for core but I don't think anyone likes using util.inherit(), worth a try ;D

Emitter(Foo.prototype);

[indutny]

We're relying pretty heavily on util.inherits in core, and I don't think that this is really necessary for us. But user-land module can certainly do it. And most importantly, you can use any inheritance mechanism with EventEmitter's constructor, so I think it is too much to have one more in core.