RePub PreProc should do all work in a tmp dir

[danh]

RePublisherPreProcess sometimes runs one error or another,
and then when you try to run it, it refuses because the
proxy directory already exists.

It would be better if it did all processing in a temporary
directory, and when done, moves the results to the
book directory. Then at least if it had problems, it could
be re-run more easily.