A faster, better link shim

Introduced a new link shim implementation that pre-checks all links on a page for maliciousness before they are sent to the browser, rather than checking only clicked links. This change eliminates the extra round trip to Facebook's servers for each click, saving approximately one second per external link click. Leveraged the 'meta referrer' tag (available in Chrome 17+ and Safari 6+) to restrict the amount of source URL information shared with external sites via the Referer header, balancing the need to indicate traffic origin with privacy concerns. For browsers not supporting 'meta referrer', the older, slower system is used.