Move BW::UIAlertView to silent beta

Per our discussion from rubymotion/BubbleWrap#229, this patch moves `BW::UIAlertView` to silent beta.  I want to be absolutely sure everyone is happy with the callback block arguments before publishing this API.

I plan on experimenting with wrappers, for additional UIKit classes, that implement their own delegate protocol.  My hope is that patterns will emerge.  Stay tuned!