[Ready for Merge] Implemented a FlipView control.

Implemented a simple FlipView control based on the [one](http://msdn.microsoft.com/library/windows/apps/BR242678) in Windows Store Apps (Windows 8/RT Metro Apps).

Based on the [Selector](http://msdn.microsoft.com/en-us/library/system.windows.controls.primitives.selector.aspx) control (like it's W.S.A. counterpart) and uses the [TransitioningContentControl](https://github.com/MahApps/MahApps.Metro/blob/master/MahApps.Metro/Themes/TransitioningContentControl.xaml) at it's core.
![m2](https://f.cloud.github.com/assets/251501/558328/336a6e5e-c415-11e2-93d4-48ebd83ecab3.png)
![m1](https://f.cloud.github.com/assets/251501/558329/336ec6ca-c415-11e2-9c8d-e95f06ff040f.png)

- [x] Implement base control.
- [x] Implement a banner.
- [x] Change arrows.
- [x] Implement any feedback provided by others: (See below)
- [x] ~~Don't show banner if ```BannerText``` is null.~~ Invalid because FlipView is trying to emulate it's W8 counterpart.
- [x] The appropriate control button disables if it cannot be used depending on ```SelectedIndex```.
- [x] Further emulate Windows 8 behavior by keeping Banner shown as long as ```IsBannerEnabled``` is ```true```.