Dismiss activity popover with action button.

This commit adds the ability to dismiss the activity popover by tapping the
button that presented it. According to [the HIG][1]: "A popover that provides
menu or inspector functionality should close when people tap anywhere outside
its bounds, **including the control that reveals the popover**." (emphasis
added.)

I didn't add an explicit check that the interface idiom is iPad because `popoverVisible` is `NO` (and the button cannot be tapped) when the activity view controller shown on iPhone.

Good work by the way. PBWebViewController looks great.

[1]: http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/UIElementGuidelines/UIElementGuidelines.html#//apple_ref/doc/uid/TP40006556-CH13-SW40