issue:
    canvas pans when tap on touch screen

reason:
    onTouchStart calls onMouseDown which registers 'mousemove' event handler.
    when I tap on touch screen, even though browser did not dispatch 'touchmove' event, it dispatched 'mousemove' event.
    therefore, the 'mousemove' handler pans canvas.

fix:
    stop onTouchStart handler registering 'mousemove' handler as onMouseMove will be properly triggered by onTouchMove
Merge pull request #168 from ethanli83/fixtouch

canvas pans when tap on touch screen