Added an indicator of duration to the Firefox extension visualizations

Added an indicator of how long each request took to
process. I did it in the easiest possible way. It would probably be better to
adjust all visualizations to have relative scale, so that for example a 1ms
request would look small next to a 100ms request, but really doing that right
would take a disproportionate amount of time.

Author: @IceCreamYou
Fixes #261
URL: https://github.com/twitter/zipkin/pull/261