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/qt/paymentserver.cpp

@@ -44,6 +44,7 @@
 #include <QUrlQuery>
 #endif
 
+using namespace std;
 using namespace boost;
 
 const int BITCOIN_IPC_CONNECT_TIMEOUT = 1000; // milliseconds