MT#16473 Relicense to GPL-3+

Use the opportunity to switch to a machine readable copyright format,
and update copyrigth years.

See <https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/>.

Change-Id: Id237ea14990ac6190665a27a1cb0c287184f0ba1

debian/copyright

@@ -1,7 +1,23 @@
-Upstream Author: The Sipwise Team - http://sipwise.com/
-Copyright:       Copyright (c) 2013 Sipwise GmbH, Austria
-License:         All software included in this package is
-                 Copyright (c) Sipwise GmbH, Austria.
-                 All rights reserved. You may not copy, distribute
-                 or modify without prior written permission from
-                 Sipwise GmbH, Austria.
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Source: https://www.sipwise.com/
+Upstream-Contact: Sipwise Development Team <support@sipwise.com>
+
+Files: *
+Copyright:
+ Copyright © 2013-2016 Sipwise GmbH, Austria
+License: GPL-3+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+ .
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+ .
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see <https://www.gnu.org/licenses/>.
+Comment:
+ On Debian systems, the full text of the GNU General Public License
+ version 3 can be found in the file '/usr/share/common-licenses/GPL-3'.