Added automatic count down progress timer option.

I was working with a system call in MCNearbyServiceBrowser called invitePeer:toSession:withContext:timeout, and I show the MBProgressHud while waiting for the peer to accept the invite.  I wanted to show the progress hud with the MBProgressHUDModeDeterminate style and have it atomically count down until the timeout happens.  So I added some methods to MBProgressHud so it can automatically countdown to a specified time.  Note: It is still the responsibility of the caller to close/hide the HUD after the timeout.