Move away from old twisted.python.plugin API.

Not bothering with the new-style twisted.plugin API right now, doing
explicit imports. This destroys some of the modularity, but that can
be fixed later if wanted.
Remove unused imports.
Make Message-IDs be unique.
Make the mimey formatter register properly.

It still doesn't work right, but that's for a different day.
Fix race on stopping a reactor before it starts.
Kludge unicode "middle dot" in titles to act as folder separator.

The code structure isn't prepare to handle real folders in XBEL input,
this gets to the same end result right now.
Typo.
Guard against newlines in subject etc header fields.

Way too ugly to live but the Python 2.x libraries aren't any better.