Add support for MSPointerEnter/MSPointerLeave events

Adding a few lines of code to add support for MSPointerEnter/MSPointerLeave in IE10, which is identical in function to mouseenter/mouseleave.  The Pointer Events spec (implemented in IE11) offers native pointerenter/pointerleave, but there's a gap in support for IE10.  To be consistent with the naming conventions used for pointer events in IE10, I've implemented this with the MSPointer* prefix and camel-casing.

Wasn't sure how to best manage the tests, so I just copied the existing mouseenter/mouseleave manual tests and modified them for MSPointerEnter/MSPointerLeave.  Let me know if you'd like something else done there.

(@tilomitra, this is the PR I previously suggested to you over email.)