Restore "content_type" kwarg to Comment.new_reply().

Restore "content_type" kwarg to Comment.new_reply().

It had been removed in
    revno: 473.1.43
    committer: W. Trevor King <wking@drexel.edu>
    branch nick: be.restructure
    timestamp: Mon 2009-12-14 07:37:51 -0500
    message:
      Transitioned comment to Command format
when we pushed unicode encoding/decoding back to the Storage backend.

However, with the addition of libbe.util.id.short_to_long_text(),
we need it again.

Also add a Doctest showing a non-text/* comment, so utilities dealing
with them can see what they'll be working with.