Initial commit of the Engine classes. GameEngine is the game thread file that holds the game loop. EngineTower is the class from which tower objects are constructed. EngineTowerList is the data structure to hold tower objects. EngineMonster is the class from which monster objects are constructed. Monster objects are stored in the EngineMonsterList. EngineEvent is the class from which event objects are constructed. EngineEventList is the data structure that holds EngineEvents. Also updated engine.pro file to reflect added classes.