Allow stacked bar chart segment spacing to be adjusted

By default, Gruff puts 1px spacing between each segment of a stacked bar chart. This commits allows that number to be adjusted.

Here's the default spacing:

![stacked_bar_keynote_1px](https://cloud.githubusercontent.com/assets/3478/3245524/23b35ae8-f173-11e3-8868-c1d6e1d228bc.png)

And here's the same graph with the segment spacing set to 0:

![stacked_bar_keynote_0px](https://cloud.githubusercontent.com/assets/3478/3245531/42a0bb76-f173-11e3-80f2-48f6895b64a4.png)