Grouped visualization for stacked bar charts

[alan-unravel]

Allows for passing of a renderType for bar-charts rendering too a grouped bar chart if "group" or a stacked bar chart if "stack"

[gordonwoodhull]

@Sid0403, @MsJR, I've also created a branch for this alternate candidate implementation for grouped bars.

Branch: https://github.com/dc-js/dc.js/tree/kavanagh-grouped-bar
Zip file: https://github.com/dc-js/dc.js/archive/kavanagh-grouped-bar.zip
Example: https://github.com/dc-js/dc.js/blob/kavanagh-grouped-bar/web/examples/grouped-bar.html

The interface is slightly different: you set .renderType('group') instead of groupBars(true), and there doesn't seem to be a way to set the gap between groups. Documentation

I notice that bar labels (which were implemented after this PR) don't work - they should be easy to fix but I couldn't figure it out with a quick look.

Trying out this implementation and reviewing it here will be of great help. We'll merge the best features from each of the implementations.