Added support for a minimum duration for a toast notification

You can now set a minimum duration for the notification to display.  This allows you to keep notifications visible even after the network calls have completed, but still afford you the luxury of calling dismiss after the time interval has passed if you need to.  For instance: I make  network call that I expect to take 1 second, but I want the notification to be visible for 3 seconds.  With this method I can set the minimumDuration to 3.0, and then in my completion block for the network call I can still call dismissNotification.  The notification will still dismiss after 3 seconds.  If the network call takes 5 seconds (poor internet connection) then the dismiss won't get called until the completion block is called at 5 seconds.