Fixes suggested by mistic100

Ahh, yes, I see the problem with the way getScrollbarWidth() is
incorporated.  Perhaps I’m hacking together this fix a bit too quickly.
:)  The scrollbar width is now calculated in the plugin’s init.  Thanks
for noticing that.

Also, I’ve updated getScrollbarWidth() to use Ben Alman’s
implementation
(http://benalman.com/projects/jquery-misc-plugins/#scrollbarwidth)

As far as I can tell, Firefox provides an incomplete implementation of
scrollLeft().  Without it, I’m not sure how to implement this
functionality.  The good news is that it seems to be fixed in FF 27
(https://bugzilla.mozilla.org/show_bug.cgi?id=717878).  I tested this
existing implementation against Firefox Nightly (29.0), and it seems to
work as expected.