ComponentGroup has chainable addComponent call
Add unit tests for new ComponentGroup behavior (adding new components)
Merge branch 'truncate-text-to-utils' into add-legend
Add ColorScales
Add utility for getting the text height in a text element
Add the Legend class. It draws a legend.

Still basically a prototype, it could probably be further improved, but it works.
Minor fix, remove the svg at the end of the last legend test
Some code cleanup
Have measurement text include ascenders and descenders
Good it delete old code
Merge pull request #126 from palantir/add-legend

Add legend