Add Safari version

I added a Safari version. A few limitations in comparison to the Chrome extension:

- The favicon isn’t set as the Safari extensions API doesn’t expose the favicon’s url like Chrome’s API does. However, I don’t consider this as a big problem as Safari doesn’t display the favicons in the tab bar anyway.
- The moon in the tabs’ titles may get truncated as [Safari truncates repetitive titles](http://developer.apple.com/library/safari/documentation/UserExperience/Reference/SBrowserTabClassReference/SafariBrowserTab.html#//apple_ref/javascript/instp/SafariBrowserTab/title).

Additionally, I added a short section in the `README.md` explaining the directory structure and the “build” process. Basically, all shared files are stored in `lib/` which is symlinked to the browser-specific folders simply by running `make`.