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/walletmodel.cpp

@@ -24,6 +24,8 @@
 #include <QSet>
 #include <QTimer>
 
+using namespace std;
+
 WalletModel::WalletModel(CWallet *wallet, OptionsModel *optionsModel, QObject *parent) :
     QObject(parent), wallet(wallet), optionsModel(optionsModel), addressTableModel(0),
     transactionTableModel(0),