Hide horizontal scrollbars but still allow scrolling

Scrollbars are hidden by default in OSX, so this should make no
difference for them
For other systems, they are plain ugly, so it might be better to hide
them
Horizontal scroll is still possible

Solution was based on the answer given here:
http://stackoverflow.com/questions/5820304/hidden-scrollbars-in-firefox-allows-scrolling-but-just-no-scrollbar