Add a cover background view

Let the toast act as an alert replacement, by blocking interaction until the notification is dismissed.
A minor replacement until  #22 is tackled.

Adds 2 new option keys: `kCRToastShowCoverBackgroundKey (BOOL)` and `kCRToastCoverBackgroundColorKey (UIColor)` 
Sets the defaults, and animates the background in/out using `animateInTimeInterval` and `animateOutTimeInterval`