Add Bullet chart to core dc.js library

Add Bullet chart to core dc.js library
http://dc-js.github.io/dc.js/docs/html/

Reference:
http://nvd3.org/examples/bullet.html

[gordonwoodhull]

There are two very different implementations of this in PRs #692 and #923. Again, they need tests to be merged. I am somewhat more inclined toward #923 because it is based on the d3-plugin instead of making the bar chart more complicated.