Create Customize group

Add jabber-otr-directory to it.
Handle message history

Don't log cryptotext to history.

Default to not log cleartext messages from encrypted sessions.  This can
be changed by setting jabber-otr-message-history, or individually for a
chat session.

This requires the patch legoscia/emacs-jabber@262602e to inhibit logging
of incoming encrypted messages.

Closes #3.