Make iScroll's defaults configurable

This simple patch makes iScroll's default options configurable on a page-level basis.  For example, if I want the scrollbars on all scrollers on my page to fade (even on Android), I could just set:

    iScroll.defaults.fadeScrollbar = true;

...and now all `new iScroll`s will have that option set to `true` without having to pass it to the constructor each time.