[tei-council] postscript scripted

[Syd Bauman]

I have put the new element for encoding postscripts into the
Guidelines. Since no one spoke up preferring <ps> to <postscript>, I
have used the name <postscript>.

I'm afraid there were two egregious errors in the free-standing ODD I
created which no one noticed:

* examples used value= (instead of when=) on <date>

* content model of <ps> was the no-longer existing macro.componentSeq
  (I didn't even notice this myself until validation of P5 failed!)

Both have been fixed.

Note: for now <postscript> is a member of model.divBottom, as I
couldn't bring myself to put it into the mis-named model.pLike.back
until that is fixed. I'm off to have lunch first, but then plan to
get to that this afternoon.