Add a cover background view

[mmmilo]

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

(Closed previous one as I didn't create a branch)

[cruffenach]

This looks great @mmmilo. Will test locally and get it in soon. Thanks!