Adding the subtool rtcBot report visualizer

This tool for visualize the output reports of rtcBot by calculating
the average and max of a specific stats and plot the output.

R=andresp@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23169004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7519 4adac7df-926f-26a2-2b94-8c16560cd09d