Adjust license file to reference BSD-3-clause.

All contributors agreed upon relicensing their contributions to the
BSD-3-clause license (http://opensource.org/licenses/BSD-3-Clause).

See terrestris/shogun#55 for discussion
and the respective agreements.

LICENSE.md

@@ -1,3 +1,46 @@
-SHOGun is licensed under GPL v3
+# Abstract
+
+SHOGun is licensed under the 3-clause BSD license, which is sometimes called 
+'BSD New' or 'BSD Simplified': http://opensource.org/licenses/BSD-3-Clause.
+
+# License
+
+    Copyright (c) 2012-2014, terrestris GmbH & Co. KG
+    All rights reserved.
+
+    Redistribution and use in source and binary forms, with or without
+    modification, are permitted provided that the following conditions are met:
+
+    1. Redistributions of source code must retain the above copyright notice,
+       this list of conditions and the following disclaimer.
+    2. Redistributions in binary form must reproduce the above copyright notice,
+       this list of conditions and the following disclaimer in the documentation
+       and/or other materials provided with the distribution.
+    3. Neither the name of the copyright holder nor the names of its 
+       contributors may be used to endorse or promote products derived from this
+       software without specific prior written permission.
+
+    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+    AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+    ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+    LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+    CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+    SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+    INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+    CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+    ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+    POSSIBILITY OF SUCH DAMAGE.
+
+# Note for client-side code
+
+    The client-side code of SHOGun partly depends on the ExtJS-framework (see
+    http://www.sencha.com/products/extjs, available separately under various
+    licensing options: http://www.sencha.com/products/extjs/licensing); it is
+    released in accordance with Sencha's Exception for Development Version 1.04
+    from January 18, 2013 (see http://www.sencha.com/legal/open-source-faq/
+    open-source-license-exception-for-development/).
+
+
+
 
-http://www.gnu.org/licenses/gpl-3.0