UC 54 - Reenabling support for some FBXML tags inside article content

This functionality existed in the FBConnect extension but hadn't yet been ported over to the new FacebookClient extension. I simplified the code a bit and removed support for any tags that we no longer want to support. Going forward, we're only supporting ```fb:recommendations``` and ```fb:like-box```.

The basic functionality of the parser hook is to load the facebook library only when there's an FBXML tag on the page.