Add md5 testing to netplay

Allows to test current game, an arbitrary game or the sdcard of all players
at once.

Source/Core/Common/MD5.h

@@ -0,0 +1,13 @@
+// Copyright 2016 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <functional>
+#include <string>
+
+namespace MD5
+{
+std::string MD5Sum(const std::string& file_name, std::function<bool(int)> progress);
+}