Report build time in a relative fashion

[anatoly techtonik <techtonik@…>]

It is hard to understand when the build was done if you don't know your time zone relative to server zone. It will be helpful if this information was available somewhere (from tooltip?) as "completed 1 hour, 23 minutes ago"

See ​http://codereview.appspot.com/ for an example of such timestamps provided by Django's timesince filter. ​https://docs.djangoproject.com/en/dev/ref/templates/builtins/#timesince