signature algorithm for old pyOpenSSL.

Fixes #7641

add configuration option tls_version

Related to
https://python-nbxmpp.gajim.org/ticket/17

Fixes #7628

add a <private> element to message stanza when message carbons is enabled and message is encrypted. Fixes #7616

http://docs.python.org/3/library/logging.html

  Note: There is an obsolete method warn which is functionally identical to
  warning. As warn is deprecated, please do not use it - use warning instead.

I have submitted the 'tls_version' to the incorrect default branch.
The correct submit was done in changeset 15331:b5831b64e5d2 ,
branch gajim_0.16 .

More on the effect of TLS tickets:
https://media.blackhat.com/us-13/US-13-Daigniere-TLS-Secrets-Slides.pdf

Fixes #7638

The prime is: 2^4096 - 2^4032 - 1 + 2^64 * { [2^3966 pi] + 240904 }

RFC3526 specifies a generator of 2.

The generation of the group is described in RFC 2412.

Fixes: #7644

add configuration option tls_version

Related to
https://python-nbxmpp.gajim.org/ticket/17

group is prefered.
Fixes #7615

Fixes: #7642

cleanup of jingle_transport.py

https://trac.gajim.org/ticket/7599

https://trac.gajim.org/ticket/7599

Correctly verify remote SSL certificate.