Increment MIT Licence copyright header year on files modified in 2016

Edited via:

$ contrib/devtools/copyright_header.py update .

contrib/devtools/check-doc.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2015 The Bitcoin Core developers
+# Copyright (c) 2015-2016 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 

contrib/devtools/optimize-pngs.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2014-2015 The Bitcoin Core developers
+# 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.
 '''