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. 

This patch also
 - fixes the location of axis labels to take into account margins
 - adds an ordinal bar chart example
 - cleans up the boxplot just a bit.
 - extends the xAxis along the full length of the chart, as per many user requests
 - adds `.barPadding` and `.outerPadding` to configure rangeBands