iOS 8 compatibility (iOS 8 only!)

I fixed issue https://github.com/problame/CSNotificationView/issues/35 which happened as a result of a IMHO not so clean solution for blur views. In my fix I am using the `UIVisualEffectView` and `UIBlurEffect` classes introduced in iOS 8. As iOS 7 doesn't include those classes (AFAIK) this will only work on devices using iOS 8 or later.