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/miner.cpp

@@ -11,6 +11,8 @@
 #ifdef ENABLE_WALLET
 #include "wallet.h"
 #endif
+
+using namespace std;
 //////////////////////////////////////////////////////////////////////////////
 //
 // BitcoinMiner