Fix race condition in Chrome 28

As discussed in #22, a lot of ChromeLogger messages seem get lost in Chrome 28.  

As far as I can tell, Chrome 28 will often capture WebRequests and fire 'onResponseStarted' before our target page's `log.js` content script has had enough time to bind a listener to the extension's `header_update` messages.  This PR captures any "lost" `header_update` messages that don't receive a `done` response, and queues them up until the content script is ready for them.