Use UIBlurEffect for MRProgressOverlayView

If at least iOS 8 is set as allowed version, and UIVisualEffectView is available
Don't animate alpha directly on UIVisualEffectView
Use a mask to apply cornerRadius to UIVisualEffectView
Touch Images.xcassets with Xcode 6
Provide a Launch Screen File to use full resolution for Example app
Merge pull request #51 from mrackwitz/features/ios8_uivisualeffectview_support

Features: iOS8 UIVisualEffectView Support