First pass at some nit stats

Very simple first pass at some nit stats. This commit:

 * Includes the Chart.js library (MIT Licensed)
 * Creates a new route /:username/nitstats for viewing nit statistics
 * On the above page, displays a line graph of nits picked and received over the past 30 days.

There's obviously a ton more that could be done but I figured this would at least get it started :-)

In response to issue #975.