Track worker behaviour(timing for now)

Added JQuery code to track worker behavior.

Modified Django model to store metrics(Currently store in the 'metrics' field of the CrowdWorkerResponse model).

I have tested this by using the DHC to test the crowd server alone, using psql to see if metrics have been stored into the database and running the CrowdDemo. 
We also might as well record total time on the page here, right? I don't think it's necessarily the sum of all the values in the `time` dictionary, so we should explicitly track it.