Allow default event handling on excluded classes

[udevnl]

Hi! First I want to say I really like how neat and plug-gable angular-drag-scroll is!

While integrating it with UI-TREE we wanted the background to be drag-gable which worked perfectly.

As an addition we wanted the tree nodes to be movable by clicking on these nodes and dragging them around. To get this done we excluded the tree nodes classes, this worked to.

However, when dragging the nodes outside of the window I expected the browser to begin scrolling (as there where scrollbars). This did not happen and after investigation I found the cause.

I created a similar titled pull request which allows angular-drag-scroll to allow default event handling on excluded classes. This way excluded classes keep default behavior.