OTR support for emacs-jabber; work in progress.

How to use it:

1. Install python-potr.  Run:

   sudo pip install python-potr

   or see https://github.com/python-otr/pure-python-otr .

2. Load jabber-otr.el.

3. Open a chat buffer (either by an incoming message, or by C-x C-j C-j).

4a. If the other contact activates OTR, the chat buffer should switch
    to encrypted mode automatically.

4b. To request OTR, type M-x jabber-otr-encrypt.

5. Send and receive messages as usual, more or less.

TODO: verify signatures, improve UI, handle HTML.