Feature: time blocks can start and end at some number of minutes/seconds past the hour

Our team uses jQuery.Gantt to plot timelines that represent work being done on a system. Some are very short, some long, and sometimes multiple of them start and finish within an hour, so, we wanted the visualization to be more exact than nearest-hour.

This update adds a chart option called showFractionalHours. When set to true, and when the scale value is "hours", bars will start and end at some pixel distance from the hour mark, proportional to the minutes/seconds/milliseconds past. This involved a slight change in the CSS classing applied to header elements. There is a test page for the feature.