Remove using namespace std from header file

It's considered bad form to import things into the global namespace in a
header. Put it in the cpp files where it is needed instead.

src/bitcoin-cli.cpp

@@ -12,6 +12,7 @@
 
 #include <boost/filesystem/operations.hpp>
 
+using namespace std;
 using namespace boost;
 using namespace boost::asio;
 using namespace json_spirit;