Step 13

Continuing from [step-12](https://github.com/angular/angular-phonecat/pull/50):

Components should be able to expose a custom events API.
I implemented that by binding a callback function (onimagechange), but I'm not sure if that's the right way to do it.

Anyway, if you add directives to the tutorial, please also add an example of how to implement custom events on components.

Thanks!! ^^