fixes for work with hidden divs and multiple infilite scrolling tabs

[nitr0man]

jscroll fails for work with hidden divs (some switching tabs with scroll in ui). this patch should fix it.

P.S. $(object).jscroll.destroy() doesn't work as expected (it destroys other istance), so I used hack in my code. I'm not too familiar with jquery internals, maybe you can fix this behavior.