run script earlier, different image page heuristic and ported to new manifest version

I changed a lot of things, don't know if you would want to merge this. First I ported it to the new manifest version 2, then I use the chrome.storage API to store the settings (can be directly accessed by content scripts) and I use chrome.webRequest to match image pages so I can run the content script earlier. I pulled in the options.css because the common sub repo seems not to have proper styles for this extension's option page and it neither has the referred commit (was it not pushed?) and I don't need common/config.js anymore (because of chrome.storage). I also made some tiny improvements to options.html: flash a "Options saved." message and use HTML5 color input.