Fix for iOS8 UIActivityViewController Popover Presentation on iPad

Tapping the Action button in iPad on iOS will generate a crash. This is because UIActivityViewControllers are presented in a UIPopoverController in iOS8. So, they need to be anchored. Added code to access the popover to set the anchor point.

See: http://stackoverflow.com/questions/25644054/uiactivityviewcontroller-crashing-on-ios8-ipads