Improvements and docs for bullet charts

Bugfixes, improvements, and documentation for bullet charts. Changes are broken out into multiple commits with detailed summaries.

You can see the changes at work in [this bl.ocks](http://bl.ocks.org/mgold/0f1d3667b74ea5616d9f); be sure to open in a new window.

Question: what would it take to move bullet charts into d3.format and out of the plugin repo?