Do speed detection with responseEnd, so it's measuring more than just latency

This was measuring the latency of the request/response rather than any bandwidth.

Adjust this to include the time it takes to download HTML to see if this makes our graphs any more reasonable. Current graph:

![Screen Shot 2013-04-02 at 17 35 21](https://f.cloud.github.com/assets/30920/329467/1ba44dc8-9bb4-11e2-8d0d-112379a0a873.png)

cc @phamann