Increment MIT Licence copyright header year on files modified in 2016

Edited via:

$ contrib/devtools/copyright_header.py update .

src/base58.cpp

@@ -1,4 +1,4 @@
-// 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.