Capture pointers so we get events even if the pointer leaves our UIElement. Fixes #1198
Docs say we should listen to PointerCanceled also

http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.uielement.pointercanceled
Merge pull request #2746 from danzel/win8-input

Handle touches going off our UIElement in Win8