Handle touches going off our UIElement in Win8

See #1198 for the bug details.

There are 2 possible ways to fix this:
 - Listen to PointerExited events and count these as a PointerRelease.
 - 'Capture' the pointer when it is pressed, this means that we get the events for it even if it is dragged off our element. I chose this fix.

I also added listening to the PointerCanceled event as we don't get PointerReleased events in certain other situations

Docs:
http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.uielement.capturepointer
http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.uielement.pointercanceled