Uses CSS positions instead of margins to position item bars.

Fixes taitems/jQuery.Gantt#13.
Re-fixes mousewheel scrolling in FF

Previous "improvement" from 2f6aabfa1d was incomplete in FF (at least)--my bad!
replaces deprecated jQuery.support.boxModel reference

jQuery.support.boxModel was removed in 1.10/2.0 .
See http://bugs.jquery.com/ticket/13743 .