Adding change and change:attr events

I'm booting a `$scope` into each model so we can start using models as event buses.

``` coffee
video = new BaseModel()
saveVideoAsync().then (videoUrl) ->
  video.set(url: videoUrl)

# elsewhere
video.on 'change:videoUrl', (video, newUrl) ->
  showSavedView(newUrl)
```

any chance we get an `off` method in here in the near future? I know there are a few places where I'm using singleton models and this binding will explode if we keep persisted models around with dead function references...