First try for UIVisualEffect on SVProgressHUD

This is a PR using the latest iOS 8 SDK classes (`UIBlurEffect`, `UIVisualEffectView`, ...) to allow for live blur. There is one downside, due to the use of these classes: the SVProgressHUDBackgroundColor has no effect anymore, can not be used. However we could provide access to UIBlurEffectStyle which allows for  light white, white and black background with blur. Is this sufficient? 

@samvermette and others: What's your opinion on this?