Add the OpenSSL exception

The GNU GPL and the OpenSSL license are incompatible. Some distributions
take a hardline stance and do not consider OpenSSL to be a systems library
(which would permit the usage/distribution of OpenSSL) when distributing
apps such as OfflineImap from the same repository. In order to solve these
distributions dilemma, we add the OpenSSL exception to our GNU GPL v2+ license.
This allows for unambiguous use/distribution of our GNU GPL'ed application
with a python linking to openssl.

Consent of all contributors has been requested via email by
Sebastian@SSpaeth.de. With very few exceptions of minor contributions (which
might or might not by copyright-worthy) all past contributors have consented
to adding the OpenSSL exception. None of the replying authors has disagreed
with adding the exception.

The corresponding issues at question:
 https://github.com/OfflineIMAP/offlineimap/issues/104
 Debian bug #747033

We are still missing consent from:
1	Asheesh Laroia
2	Bart Kerkvliet
4	Daniel Burrows
5	David Favro
1	David Logie
1	Eric Dorland
1	Ethan Schoonover
49	Eygene Ryabinkin
1	Loui Chang
1	Luca Capello
1	Michael Witten
2	Mike Dawson
1	Peter Colberg
1	Scott Henson
1	Tom Lawton
1	W. Trevor King
2	X-Ryl669
1	buergi
2	dtk
5	mj

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

COPYING

@@ -348,3 +348,17 @@ proprietary programs.  If your program is a subroutine library, you may
 consider it more useful to permit linking proprietary applications with the
 library.  If this is what you want to do, use the GNU Library General
 Public License instead of this License.
+
+----------------------------------------------------------------
+In addition, as a special exception, the copyright holders give
+permission to link the code of portions of this program with the OpenSSL
+library under certain conditions as described in each individual source
+file, and distribute linked combinations including the two.
+
+You must obey the GNU General Public License in all respects for all of
+the code used other than OpenSSL. If you modify file(s) with this
+exception, you may extend this exception to your version of the file(s),
+but you are not obligated to do so. If you do not wish to do so, delete
+this exception statement from your version. If you delete this exception
+statement from all source files in the program, then also delete it
+here.