add response time to hurls

This pull request adds `response_time` to hurls, which is measured by [Curl::Easy.total_time](http://curb.rubyforge.org/classes/Curl/Easy.html#M000124).

![Screen Shot 2013-01-17 at 5 46 47 PM](https://f.cloud.github.com/assets/1109058/77036/f388c9be-6110-11e2-8243-80488e483a55.png)