Use CBitcoinAddress instead of string/uint160

Instead of conversion functions between pubkey/uint160/address in
base58.h, have a fully fledged class CBitcoinAddress (CAddress was
already taken) to represent addresses.

src/key.h

@@ -220,6 +220,11 @@ public:
             return false;
         return true;
     }
+
+    CBitcoinAddress GetAddress() const
+    {
+        return CBitcoinAddress(GetPubKey());
+    }
 };
 
 #endif