Facebook comments should now be specific to each page. Fixes #821.
Merge pull request #824 from afred/fix-facebook-comments

Facebook comments should now be specific to each page. Fixes #821.

Error is the normal blog failure. (TODO)

@avatousiou: could you look at http://stackoverflow.com/questions/2165665? It would be nice to have this a little more general, so that it doesn't depend on a `catalog` path and a `@document` field.

I'll merge this, but unless you see a problem with that tweak, it might be good.