Mousewheel: added debounce delay - for the duration of mousewheelDebounc...

...e (600ms by default) all additional mousewheel events will be ignored - fixes the problem that when using e.g. an apple track pad and scrolling down >5cms with acceleration, swipeNext gets triggered 3-5 times instead of just once.

Ideally we'd only handle this without any delay and just wait until the touch has stopped but this fixes my problem very well so far and is a relatively easy fix.
Also for simplicity's sake I just imported Underscore.js's debounce function - maybe not the best idea - feel free to handle this differently.