Display the runtime of each job on the history page

I wanted to get a sense of how long each of my jobs were running historically to help fine-tune my scheduling. I extended the history middleware to keep track of how long each job takes and store that along with the history entry, and then display it as a column in the history view.


![image](https://cloud.githubusercontent.com/assets/792845/4158697/644f1554-3491-11e4-9530-c9a01f7ae3b9.png)