Added Y.App.Transitions \o/ This is an opt-in feature that gets auto-mixed onto `Y.App`. Transitions will only apply to modules which support native CSS3 transitions. To handle this, `app-transitions` acts as the trigger to test and load, `app-transitions-css` and `app-transitions-native` modules. These two provide the _real_ implementation.

Moved app-transitions.js under app-extensions/ dir.

App.Transitions is now opt-in. Changed the transitions support that this adds to Y.App to be opt-in, this is because it is auto-mixed into Y.App. To add these transitions, it now overrides `_uiSetActiveView()` which is provided by Y.App.Base.

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.