Added step-line, step-area, and arc-gauge charts, modified area charts to group without exceeding total.

For our purposes we needed to be able to show a step-line chart with area fill. We also wanted to show arc-gauges like  that from JustGage(.com) but also include negative values on the gauge. We decided to build it into c3 instead of adding another library and modify that.

We attempted to integrate changes from over the weekend (0.1.42) prior to sending this request, but ran into issues with the change to lineOnMain and lineOnSub. We prefer initializing a drawLine, as you did with drawBar, from withing the redraw function. This allowed for the line on the main chart to change with the selection on the sub chart and click draging. I noticed you have the same bug in your sub chart example with the release of 0.1.42. (http://c3js.org/samples/options_subchart.html). 

When couldn't accept not having the framework of a chart when no data was supplied so we added functionality to define a "No Data" view when none was supplied.

Some, of the key functionality of JustGage we included in the arc-gauge
Level based coloring. 
Min/Max labeling.
Background arc coloring.
Arc gauge width.