Change ZNC license to Apache 2.0

The following people agreed with the change, in alphabetical order:
(people who approved in several ways are listed only once)
By email:
- Adam (from Anope)
- Austin Morton
- Brian Campbell
- Christian Walde
- Daniel Holbert
- Daniel Wallace
- Falk Seidel
- Heiko Hund
- Ingmar Runge
- Jim Hull
- Kyle Fuller
- Lee Aylward
- Martin Martimeo
- Matt Harper
- Michael J Edgar
- Michael Ziegler
- Nick Bebout
- Paul Driver
- Perry Nguyen
- Philippe (cycomate)
- Reuben Morais
- Roland Hieber
- Sebastian Ramacher
- Stefan Rado
- Stéphan Kochen
- Thomas Ward
- Toon Schoenmakers
- Veit Wahlich
- Wulf C. Krueger

By IRC:
- CNU
- Jonas Gorski
- Joshua M. Clulow
- Prozac/SHiZNO
- SilverLeo
- Uli Schlachter

At https://github.com/znc/znc/issues/311 :
- Alexey Sokolov
- Elizabeth Myers
- flakes
- Jens-Andre Koch
- Jyzee
- KindOne/ineedalifetoday
- Lee Williams
- Mantas Mikulėnas
- md-5
- Reed Loden

At the last few pull requests' comments:
- Allan Odgaard
- Jacob Baines
- Lluís Batlle i Rossell
- ravomavain
- protomouse

The following commits' authors didn't respond:
Trivial changes:
- f70f1086fd
- 4ca8b50e45

The changes which are not presented in master anymore:
- 5512ed2ea0
- 960a4498f7
- 0f739de2c0
- 7f53cc810b

Fix #311
Fix #218

modules/clearbufferonmsg.cpp

@@ -1,9 +1,17 @@
 /*
- * Copyright (C) 2004-2013  See the AUTHORS file for details.
+ * Copyright (C) 2004-2013 ZNC, see the NOTICE file for details.
  *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms of the GNU General Public License version 2 as published
- * by the Free Software Foundation.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
  */
 
 #include <znc/IRCNetwork.h>