multi: retain original copyright on files copied/modified from btcsuite

Early in the lifetime of the project here were a few files we either
copied entirely, or used as the basis for code within lnd. Before this
PR, this was not recognized by retaining the original copyright. With
this commit, we remedy that by explicitly noting the copyright in the
relevant files.

Fixes #423.

version.go

@@ -1,6 +1,9 @@
-package main
-
+// Copyright (c) 2013-2017 The btcsuite developers
+// Copyright (c) 2015-2016 The Decred developers
 // Heavily inspired by https://github.com/btcsuite/btcd/blob/master/version.go
+// Copyright (C) 2015-2017 The Lightning Network Developers
+
+package main
 
 import (
 	"bytes"