Adds option to plot average in boxplot, besides the median

I wrote this as I needed it and matplotlib didn't support it.

Please let me know if it looks good or if anything needs improvement.

I've attached an example of how it looks.
![united_states_averages](https://f.cloud.github.com/assets/473201/1331003/ecd14a46-3541-11e3-8ac8-a58ebcf79dc2.png)