Replace deprecated chrome.extension use with chrome.runtime equivalents (fix for Chromium on Ubuntu 13.10)

chrome.extension.onMessage and chrome.extension.sendMessage are not available in content scripts on Chromium 29.0.1547.65 (shipping in Ubuntu 13.10). This may be a bug with Ubuntu's specific build and I can't reproduce it on Chrome's stable channel but it does reveal that RES is using the deprecated extension namespace for the messaging API. See the [deprecation notice](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-extensions/7ZCpcIkJ1RM) and a related [bug](https://code.google.com/p/chromium/issues/detail?id=289776).

Possible downside is this requires Chrome >= 26 for the newer chrome.runtime namespace.