Add unfinished documentation for VapourSynth.h

Firesledge's work[1] served as starting point. Most (all?) of it got
rewritten.

The whole thing should be reviewed to make sure I didn't add any lies.

Most of the "structs" need descriptions, and two or three functions too. Firesledge left behind quite a few questions, and some of them are still there. These things are marked with "???" or "TODO".

[1] http://pastebin.com/raw.php?i=wQqCETGM