Inherit  subscriptions from prototype?

So that emitter becomes a real mixin, doesn't require `Emitter.call(this)`  and doesn't force you to pollute constructors with unnecessary subscription job.

``` javascript
function SuperDialog () {}

SuperDialog.prototype = new Dialog

SuperDialog.prototype.on('show', function () {
   // do super thing
})

// SuperDialog is a good mixin
new SuperDialog // fast
```