Change license to Apache 2.0 (#419)

Replace references to GNU Public License to Apache license in:

 - COPYING file
 - README
 - all source code below falco
 - rules files
 - rules and code below test directory
 - code below falco directory
 - entrypoint for docker containers (but not the Dockerfiles)

I didn't generally add copyright notices to all the examples files, as
they aren't core falco. If they did refer to the gpl I changed them to
apache.

userspace/engine/falco_engine.cpp

@@ -1,19 +1,20 @@
 /*
-Copyright (C) 2016 Draios inc.
+Copyright (C) 2018 Draios inc.
 
 This file is part of falco.
 
-falco 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
 
-falco 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.
+    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.
 
-You should have received a copy of the GNU General Public License
-along with falco.  If not, see <http://www.gnu.org/licenses/>.
 */
 
 #include <cstdlib>