Preliminary work on rewrite.
Update documentation, increase code consistency.
Refactor brush/zoom code.
Add ornaments to AxesChart.

Ornaments are similar objects to plotables, but do not represent plots,
rather things like text boxes that do not require scales and axes.
Add d3.plotable.TextBox ornament.
Adjust default TextBox position for new container.
Restore default margins, adding more dynamically.

The z colour scale needs a larger right-side margin to be fully
displayed, but having a chart with a large right-side margin and no
scale means lots of wasted space.
We now add more margin, if it's needed, when adding a z scale.
Merge pull request #9 from alexpearce/radical

Rewrite chart objects for simpler composition. Closes #4 , towards #3.