added bar-target-graph and some fixed to barchart.

[d2a-raudenaerde]

For creating charts in the style of bullet-charts..

[gordonwoodhull]

This is really cool. However it might take some work to adequately integrate/generalize these features, so I'm scheduling it for v2.1.

Also, it introduces a bunch of minor changes to calculations which break tests, which will need to be reviewed.

For anyone interested in this feature in the meantime, I rebased the changes and started a target-chart branch: https://github.com/dc-js/dc.js/tree/target-chart

I will occasionally update this branch to keep it in sync with master.

[gordonwoodhull]

Here is the SO question where this started: http://stackoverflow.com/questions/25565177/target-in-barchart-in-dc-js