Add delegate method barChartView:colorForBarViewAtIndex: to JBBarChartViewDataSource

Implement user-friendly method to set proper color of bar chart. 
(The same thing is already implemented for JBLineChartView lineChartLinesView:colorForLineAtLineIndex:)

Try to save great author's code style  and add documentation for this method.

Logic: Check if this method implemented and set specified color before set the default kJBBarChartViewDefaultBarColor.

Also implement this method in Example project