Error bars

This Pull Request adds native support for error bars to line charts and bar charts.

To facilitate this, an array can be passed instead of a number for a single datapoint. The average of this array will be plotted as the data point. This is compatible the Chart,js API: you don't HAVE to pass an array.

The following options are exposed:
- error: type of error bar. None (default), stdev (standard deviation), stderr (standard error) or range (full range of values from min to max)
- errorDir: direction. "Both", "up" or "down"
- errorStrokeWidth
- errorCapWidth: the ratio of the width of the error bar caps to either the width of the bars or the diameter of the points. Default: 0.75 for bar charts and 2.0 for line charts.

An errorStrokeColor can declared on each dataset. However, this is optional and by default error bars will have the same stroke color as the dataset itself.

I added new samples (bar_error.html and line_error.html) so that this should all hopefully be clear.

Please +1 if you'd like to see this natively supported in Chart.js - otherwise I'll maintain on my branch.

Will work on documentation and add commits here.