[copyright] add MIT license headers to .sh scripts where missing

Years are set according to 'git log' history

contrib/verify-commits/gpg.sh

@@ -1,4 +1,8 @@
 #!/bin/sh
+# Copyright (c) 2014-2016 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 INPUT=$(cat /dev/stdin)
 VALID=false
 REVSIG=false

contrib/verify-commits/pre-push-hook.sh

@@ -1,4 +1,8 @@
 #!/bin/bash
+# Copyright (c) 2014-2015 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 if ! [[ "$2" =~ ^(git@)?(www.)?github.com(:|/)devrandom/gitian-builder(.git)?$ ]]; then
     exit 0
 fi

contrib/verify-commits/verify-commits.sh

@@ -1,4 +1,8 @@
 #!/bin/sh
+# Copyright (c) 2014-2016 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 # Not technically POSIX-compliant due to use of "local", but almost every
 # shell anyone uses today supports it, so its probably fine