Support for the dragover event, improvements for dragenter & dragleave.

With the addition of this patch, we get closer to functional parity with HTML5 drag & drop. The patch includes:

1) Support for the ```dragover``` event, which is configured with the already existing ```enableEnterLeave``` flag.
2) Support for the ```pageX``` & ```pageY``` properties of the event object sent with the ```dragenter```, ```dragover``` & ```dragleave``` events.
3) Some sanity checks that none of these events get sent to ```null```.

These changes are motivated by my real-world usage. The ```dragover``` + ```pageX/Y``` combo allows me to give different UI hints based on where exactly the user is attempting to drop an element. The event sending to ```null``` is attempted when moving the finger out of document bounds, e.g. over the browser UI. Desktop Chrome 39 is an example browser that keeps sending ```touchmove``` events even beyond the HTML controlled viewport.