Fix bug when mouseMove calls after mouseUp in DragManager

This should never happen (because we may reset _activeTarget back after
setting it to null in _mouseUp, which can cause exceptions).

But sometimes that may happen (e.g. in Chrome on Windows), because we
actually call globalOnDragEnd on the next tick after mouseUp. So,
sometimes the sequence of calls may be:

  mouseMove -> mouseUp -> mouseMove -> globalOnDragEnd

To avoid that problem, let's just clear up _activeTarget in
globalOnDragEnd
Merge pull request #10 from astashov/fix-redundant-mousemove-bug

Fix bug when mouseMove calls after mouseUp in DragManager