all: change copyright notice to The Go Cloud Authors (google/go-cloud#306)

Added AUTHORS+CONTRIBUTORS files with the contributors already present
from the Git log.

Fixes google/go-cloud#225

internal/wire/parse.go

@@ -1,4 +1,4 @@
-// Copyright 2018 Google LLC
+// Copyright 2018 The Go Cloud Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.