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. 
Theoretically we could store any metric in here, right? No reason to limit the metrics to task_ids and timings... a map from 'metric_name' to 'metric_value' is all we need.