fix bug 769516 - Add $toc document option

Adds a "$toc" address for documents which returns only the TOC for a document.  The ticket says to return a table but that doesn't make much sense;  it's unstructured (no nesting).

The following wiki.toc method would need to be added to work with said address:

https://gist.github.com/4513464

https://bugzilla.mozilla.org/show_bug.cgi?id=769516