Allow for keyboard shortcuts to extension URLs ( Stats, Rule Manager, Settings )

Allow for keyboard shortcuts to extension URLs that can be set in chrome://extensions/ at the very bottom of the page where it says "Keyboard shortcuts."  Users would be able set whatever shortcut they would like to use.  The 'suggested_key' in the manifest.json didn't do anything for me.  Had to set it manually.

I also wasn't sure where to put the ```chrome.commands.onCommand.addListener```... so I just put it in ```start.js```.

Was thinking to put a shortcut to toggle HTTPSB On/Off like you can in that little dropdown.

![httpsbshortcuts](https://f.cloud.github.com/assets/6408935/2093908/64bee7f6-8ec1-11e3-96f1-853b96b0ab73.PNG)