Add Apache 2 license and copyright.

Add Apache 2 license and Hootsuite copyright to all golang
files. Add a disclaimer that the files have been modified
hereafter by contributors to this repo in order to abide
by the Apache 2 license requirements.

server/server_test.go

@@ -1,3 +1,16 @@
+// Copyright 2017 HootSuite Media Inc.
+//
+// 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.
+// Modified hereafter by contributors to runatlantis/atlantis.
+//
 package server_test
 
 import (