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. 
Hm, I wonder if we should normalize metrics as their own model, since we might want to query over them (e.g. 'get the average response time per task grouped by task type').

What do you think? Is there a reason you chose to keep it as denormalized JSON here?