Features: iOS8 UIVisualEffectView Support

Uses `UIVisualEffectView` in combination with `UIBlurEffect` for the blur in `MRProgressOverlayView`.

* [x] Blocked by iOS 8 GM.
* [x] Blocked by [rdar://18466755](http://www.openradar.me/18466755). (Specific to "classic" resolution on new phones.)