Add command-line option to generate treemaps instead of scatter plots

So I've been talking up this tool for a long time, and everyone says "Kerri, it sounds cool, but that graph is kinda hard to understand!"  

![scatter_plot](https://f.cloud.github.com/assets/257689/1061321/408a2852-11ce-11e3-849c-75ee07199987.jpg)

Well, to each their own! However, once I started piping the output into a treemap graph, I came to really like the clarity and "idiot light on the dashboard" quality that the treemap graph offered. 

![treemap](https://f.cloud.github.com/assets/257689/1061324/800741e0-11ce-11e3-8ca8-41380d0b26bf.jpg)

Size of each rectangle is the relative amount of churn, the color scale is complexity, from best in the project (green) to worst (red.) (I know this is rough on the colorblind, my apologies.)

Although I did rummage the files around a bit, original functionality is preserved, simply adding a ```--treemap``` option to the command line to generate treemaps instead of scatter plots.