Add dragSelection methods, ignore scrollbar clicks

Resolves #181 

While adding functionality to ignore clicks on the scrollbar, there was found to be two 'mousedown' listeners in the Drawer.  

To combine them into one 'mousedown' listener, I allowed 'mousemove' to always track a POSSIBLE drag selection, but not raise the event unless the feature is active.  

This then created the ability to modify the 'dragSelection' parameter via three new methods.


Won't work on Mac, because scrollbars are inside the element:
![Imgur](http://i.imgur.com/U32O6fY.png)

Does it work on Windows?