Fixed issue with crashing map view when tapping on a pin (#74)

This fixes issue #74: https://github.com/ushahidi/Ushahidi_iPhone/issues/74

The problem was that it was relying on the `[[button superview] superview]` layout hierarchy, which changed in iOS 7. I changed this to use the `calloutAccessoryControlTapped` instead.

Tested on iOS 6.1 and iOS 7.1.