Decoupled Updatable from Sprite#getDisplayObject

* Moved Updateable initialisation param to a separate call #onTick
* The new `#onTick(displayObject, callback)` method receives the
display object that gets attached the ‘click’ event. This allows other
mixins to add their on onTick events attached to other displayObjects.
I plan to use this on referencePositionDisplayObject and
collisionBoundingBoxDisplayObject.
Added Renderable mixin

Callbacks can be added to the `render` event.
Sprite now calls `#onRender` instead of defining `#render`.
This will allow other mixing to hook on the render event.
Merge branch 'refactor'