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. 
This feels like an odd implementation choice. You're already using mouseenter event listeners to track focus, why not just track timing inside the event functions? For example, on mouseenter, record the focus and the timestamp at which the focus started, and on mouseleave, take a new timestamp and determine how long the focus was held.

In general, setInterval is a pretty ugly solution, as it constantly churns the CPU even when the user isn't doing anything.