Implement Precompiled Headers (#591)

When applied, this will speed up compile times dramatically.

dGame/dMission/Mission.h

@@ -1,5 +1,8 @@
 #pragma once
 
+#ifndef MISSION_H
+#define MISSION_H
+
 #include <vector>
 #include <string>
 
@@ -259,3 +262,5 @@ private:
      */
 	std::vector<MissionTask*> m_Tasks;
 };
+
+#endif