Add highlighting support for mobile/touch

[hempels]

Issue #15 offers a solution for pen/drawing, but highlighting is a completely different animal. From what I've seen, capturing highlighting on mobile web requires replacing the built-in selection gestures since those aren't able to be used like they can with desktop browsers.

Without support for mobile this library's usefulness is severely limited so I would consider this feature as a relatively high priority.

[rogercorrea]

I need it also.
I'm trying adapt touchstart event for him but not work for me.

[rogercorrea]

I fixed problems and create a new fork with pull request #85 .

[bmtdh]

@rogercorrea did you ever get drawing a rectangle, line and highlighting working on touch devices? I tried your fork but these don't work. I can't see an easy way of adding it in.

Thanks,