Moved Y.App's transitions feature into an extension. A new submodule, app-transitions, has been created to contain the Y.App.Transitions extension. When this module is used, it will auto-mix itself onto the Y.App constructor. When auto-mixing features, it is important to leave the "base" class untouched. To achieve this, I followed @lsmith's method, which results in the following Y.App namespace: * Y.App.Base: The base constructor. * Y.App.Transitions: The app transitions extension. * Y.App: A subclass of Y.App.Base which extensions can auto-mix on to.