Add .barWidthAccessor to manually specify (per-bar) widths for barCharts

This is an attempt to address my question on StackOverflow at http://stackoverflow.com/questions/23764932/dc-js-bar-chart-different-widths-and-title-problems .

The width is specified in pixels rather than in the default xaxis units because I couldn't think of a way to have a sensible function that would work with time axes.