Minor optimisations.
Support negative height.

This effectively allows the vertical ordering to be reversed, as long as
you set an appropriate translate transform in the chart's parent <g>
element.
Document negative height behaviour.
Remove a few unused variables.
Fix update/exit selections.
make tooltip appearance customizable through css
Merge remote-tracking branch 'cscheid/master'
Remove a few global styles.
Add parsets.{tooltip,categoryTooltip}.

Fixes #5.