All App Framework modules now show up on the API docs overview page. Added `@module app` to all of the source files. Also included `Y.App.Transitions` in the README.

API Doc updates for App.Transitions and App.TransitionsNative.

Fix to only transition the last `activeView` change in the queue. Maintains the API contract of the `showView(view, config, callback)` by always making sure to call the specified `callback` function; but when multiple calls to `showView()` happen rapidly, they will be queued and the visual transition will only happen for the last item in the queue. Fixes #2531846

Fix for App.Transitions sliding transitions. This makes sure the views are at full opacity (1) before sliding them. The transition timings were also turned down to 0.3 seconds.

App.Transitions now handles rapid calls to `showView()`. Fixes #2531760

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.