Threaded comments

Well, they sort of work. Mostly.

Code for threaded comments. Patch for MT 3.11.

Important note: His tutorial does not include info on using TypeKey with threading (but I figured that out and will write it up eventually). Nor how to "hide" a comment while it's awaiting approval.

I'm leaving a note for the patch author to see if he's tried this yet. Meantime if things look funky, please be patient.