Fix bug when mixin dgrid/extensions/DnD and dgrid/Selection

### The problem
When mixin `dgrid/extensions/DnD` and `dgrid/Selection`; using IE10 the `mousedown` event is reported as `MSPointerDown`. Because the Selection module is not listening for this new event the Grid's selection is always empty. Because the DnD extension relies on the Grid's selection (if any) it does not work as the later is always empty.

### Tests
After testing in different borwsers (in a non-touch laptop), these are my findings about the event type reported:
##### IE (v 10.0.9200.16635, update 10.0.7)
* IE10/Standars: `evt.type = "MSPointerDown"`
* IE9/Standars 9: `evt.type = "mousedown"`
* IE8/Standars 8: `evt.type = "mousedown"`
* IE7/Standars 7: `evt.type = "mousedown"`

##### Chrome (v 29.0.1547.18 dev-m)
`evt.type = "mousedown"`

##### FireFox (v 22.0)
`evt.type = "mousedown"`

### "Fix"
Add `MSPointerDown` and `MSPointerUp` to `Selection.selectionEvents` property.

### Little investigation (in commit message)
MSPointerXXX events are inconsistent with the fact that has('touch') returs false. So in IE10 if you try to dedect a touch device 'has' reports false; but the browser fires events as MSPointerXXX. 
The feature detection is a little dirty 
`if (window.navigator.msPointerEnabled) { // Must listen for MSPointerXXX events } 
else { // Classic event types };`
or maybe 
`if (has('ie') && has('ie') >= 10) {...}`

Information source http://msdn.microsoft.com/en-us/library/ie/hh673557(v=vs.85).aspx