Add logback to licenses.

res/com/dmdirc/licences/Logback - LGPL

@@ -0,0 +1,11 @@
+Logback: the reliable, generic, fast and flexible logging framework.
+Copyright (C) 1999-2012, QOS.ch. All rights reserved. 
+
+This program and the accompanying materials are dual-licensed under
+either the terms of the Eclipse Public License v1.0 as published by
+the Eclipse Foundation
+ 
+  or (per the licensee's choosing)
+ 
+under the terms of the GNU Lesser General Public License version 2.1
+as published by the Free Software Foundation.