Execution time graph

Adds a graph to the monitor page that displays the average time between a task being created and succeeding. This time is broken down into time waiting in the queue, and execution time. 

Currently it does not count tasks that have an ETA as it was making the queue time look artificially high.

![Sample graph](https://f.cloud.github.com/assets/775741/98154/c9e80d62-6715-11e2-80a8-1b5363b8ddd1.png)