Expose stack offset

Fix #225 -
Thankfully, d3 has a percentage-based stacked column baked into the stack layout in the form of `stack.offset`. All you need to do is set the offset to expand and you get a chart like this:

![screen shot 2013-08-24 at 8-24-13 7 12 01 pm](https://f.cloud.github.com/assets/1975147/1021599/a47b941a-0d12-11e3-8a5b-6064dd7c8bfd.png)

This PR exposes the offset options. The only thing remaining is to change the yAxis format to %, but I figured that it was best to leave the format up to the user, but feel free to add if you think it should be the default format.