Add support for flotr2 as an alternative to Google Charts

I have made changes adding the JavaScript charting library Flotr2 as an alternative to Google Charts. I think it is a much better alternative when you are using it in a corporate environment with no need for external calls. Toggle between Google Charts and Flotr2 in the gitblit.properties file: web.chartType = flotr2 or web.chartType = google. The default is google.