History mode

Re-implement the history mode for debug kit.

This version works very differently than the way it has in the past and looks a bit different too. It looks like this now:

![screen shot 2014-08-23 at 6 09 14 pm](https://cloud.githubusercontent.com/assets/24086/4021076/e5b774e4-2adf-11e4-9d5b-9db151f04215.png)

After choosing a new request, all other panels will load data from that request. I'm not sure if it is necessary to show the url/time on each panel to make it clearer you are in a 'history' mode. I'm quite open to suggestions around that.