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