Add error bars, scatter plots (with trendlines), box-and-whisker plots

There's a full description here: http://www.caseyy.org/code/a-stem-fork-of-chart-js/

Some of this is a little unusual in that you can submit an array for each datapoint. The mean (or median) is calculated and plotted as the data point using an additional object (ChartCalculator) and passed along to window.Chart. This is a bit beyond just adding new chart types, but it's kind of necessary for using error bars (because you need a population to base that error bar on).

Feel free to take what you like; I won't be hurt if you reject all of it. I've really moved onto using d3 for the most part.