Visualise the event queue length on self stats page

Looks like this: 

![image](https://cloud.githubusercontent.com/assets/12306/2851980/ff355e92-d128-11e3-9fef-e5bfaafa1764.png)

You can get an idea of what it looks like in action here: 

https://www.youtube.com/watch?v=RcrlrZi1M3s

Currently updates the graph every 5 seconds. May need to make this tunable.