Add simple box plots

[grero]

This adds a basic boxplot function to Winston. I've included code to indicate notches around the median, which uses a simple analytical expression for the notch size (http://en.wikipedia.org/wiki/Box_plot). I'm mainly submitting this to get comments, as I'm sure the way I'm currently doing this is less than optimal. Eagerly awaiting any criticisms : )

[nolta]

Thanks! Just a few minor suggestions:

• I'm not sure about the name Boxplot. How about something like QuartileBoxes or TukeyBoxes?
• Needs docs. Specifically, a doc/fun/boxplot.rst file and a line in NEWS.

[grero]

Thanks for the comments I think QuartileBoxes might be a better name, I guess I used Boxplot since that's what Matlab and Matplotlib calls these kinds of plots. I actually prefer looking at distributions like this rather than as a histogram. I'll put together some documentation as well.