Change license to Apache 2.0.

Mailing list discussion was here:
https://groups.google.com/forum/#!topic/redo-list/wLMZMxtn4wo

Several more contributors replied to me personally to say that they
don't have a problem with the change (and several consider the change
to be an improvement).

The overwhelming majority of everything in the redo repo was written by
me, so I have the right to change the license unilaterally anyway,
subject to a few rules.  Since the new license actually removes
licensing/usage restrictions for everyone, this should be no problem.

redo.py

@@ -1,4 +1,19 @@
 #!/usr/bin/env python2
+#
+# Copyright 2010-2018 Avery Pennarun and contributors
+# 
+# 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.
+#
 import sys, os, traceback
 import options
 from helpers import atoi