Add an "SSL exception" to the license.

This is to clarify that mitmproxy can be distributed with OpenSSL. It's unclear
whether this is really needed, but I've had at least one request for this, and
there's a precendent in other Open Source projects.

LICENSE

@@ -1,3 +1,19 @@
+mitmproxy is licensed under the terms of the GNU General Public
+License version 3, with the following addition:
+
+  In addition, as a special exception, the copyright holders give
+permission to link the code of this program or portions of this
+program with the OpenSSL project's "OpenSSL" library (or with modified
+versions of it that use the same license as the "OpenSSL" library),
+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) provided
+under this license, you may extend this exception to your version of
+the file, but you are not obligated to do so. If you do not wish to do
+so, delete this exception statement from your version.
+
+
                     GNU GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007