add Addon.prototype.isEnabled for an addon to exclude itself from the tree

closes #2269
I don't think it's been called at this point, get's called [here](https://github.com/stefanpenner/ember-cli/blob/master/lib/broccoli/ember-app.js#L196) I was thinking it was too, thus I agree with removing the argument if we can switch it up to be like that.

Seems like the addon's might need to be removed after being initialized in `EmberApp.prototype._notifyAddonIncluded`. That way they can access the full app instance due to included being called. Seems a little dirty though