enable Pingdom RUM

Pingdom has a new real-user monitoring (RUM) service.  Enable sending
perf stats (non PII) from *all* pages to Pingdom's service, much like
we already do with Google Analytics.

I tested this locally on my machine by setting the Pingdom domain to http://class.stanford.edu:8000 and then manually setting class.stanford.edu to 127.0.0.1 in my /etc/hosts.  That seemed to work.  

@jrbl or @jbau