adding popover completion block

While the UIPopoverController API lacks completion blocks, I had a case where such a thing was useful (needed it to happen after the animations, and an in-class completion block would be the best way to accomplish that with timing and all). So, here we are.

I only added it to the `-presentPopoverFromRect:` because that's all I needed, but if this is added it'd probably be best to flesh out the other `present` methods too.