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>

README.md

@@ -17,9 +17,10 @@ documentation.
 OfflineIMAP does not require additional python dependencies beyond python >=2.6
 (although python-sqlite is strongly recommended).
 
-OfflineIMAP is a Free Software project licensed under the GNU General Public
-License version 2 (or later). You can download it for free, and you can modify
-it. In fact, you are encouraged to contribute to OfflineIMAP.
+OfflineIMAP is a Free Software project licensed under the GNU General
+Public License version 2 (or later) with a special exception that allows
+the OpenSSL library to be used. You can download it for free, and you
+can modify it. In fact, you are encouraged to contribute to OfflineIMAP.
 
 Documentation
 -------------