Snap offset

Add offset to snapped element
Chage deceleration and accelerate on multiple scroll
Change selector for snap to getElementsByClassName
_wheel function modified

Now scrollStart is called. Snap only fires at the same time as
scrollEnd.
Add option to scroll horizontal with mousewheel

If mouseWheelScrollsHorizontally is set to true, vertical mouse wheel in
a horizontal iscroll will make it scroll horizontally. If set to false,
vertical mousewheel will do nothing but instead horizontal mousewheel
(trackpad) will.