Use RangeBands for Ordinal Coordinate Grid Charts

Closes #311. We use rangeBands, but keep the gap logic for bar charts and xUnits. So we have full backwards compatibility at the moment. I'd like to expose the padding option to rangeBands instead of gap in the future and I'd also like to somehow deprecate xUnits, but this seems like a solid incremental step.

This patch also
 - fixes the location of axis labels to take into account margins
 - add and ordinal bar chart example
 - and cleans up the boxplot just a bit.
 - extends the xAxis along the full length of the chart, as per many requests on the mailing list (do we have an issue?)

Future issues:
 - Drawing the full xAxis line, brings to light that the outPadding of 0.5 a bar width is fairly large. We'll need to expose a method to customize.
 - Likewise for interbar padding