Fix: when dragging the slider, the mouse movements get attached to the s...

When the nanoScroller is created dynamicly within a select2 instance, there is an issue - when you start dragging the slider with the mouse, after releasing it (if it is not at the top or bottom) the mouse movement gets translated as scroll.

To fix it I added an event listener for `mouseleave`.

Sorry for the first one, I am new new to git and didn't see that I changed the other files. Now I have included only the `.coffee` file that can be compiled to js.