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/paymentrequestplus.cpp

@@ -17,6 +17,7 @@
 #include <QDebug>
 #include <QSslCertificate>
 
+using namespace std;
 
 class SSLVerifyError : public std::runtime_error
 {

src/qt/paymentrequestplus.h

@@ -24,7 +24,7 @@ public:
     PaymentRequestPlus() { }
 
     bool parse(const QByteArray& data);
-    bool SerializeToString(string* output) const;
+    bool SerializeToString(std::string* output) const;
 
     bool IsInitialized() const;
     QString getPKIType() const;

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

src/qt/transactiondesc.cpp

@@ -18,6 +18,8 @@
 #include <stdint.h>
 #include <string>
 
+using namespace std;
+
 QString TransactionDesc::FormatTxStatus(const CWalletTx& wtx)
 {
     AssertLockHeld(cs_main);

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),