Normalise 2DHistogram coding style, tweak legend.

Mostly formatting changes like tabs to spaces, indentation.
Changed the legend drawing code to have the scale fill the chart's
height.
Histogram is now drawn underneath the axes so the tiles don't hide the
ticks.
Update examples for tweaked 2DHistogram chart.
Rename 2DHistogram to Histogram2D.

CSS classes can't start with a number, and conventionally we give
containers a class as their chart name.
So, the rename allows us to stick to this convention.
Final Histogram2D rename changes.
Merge pull request #8 from alexpearce/2d-changes

2DHistogram tweaks