include/App.h
  - Added onClose(), onExit() methods.
  - Changed the constructor to take width, height, and title of window instead
    of an actual window.
  - Added a boolean to mark whether the App is running or not.

src/App.cpp
  - Redefined constructor.
  - onInit() now mallocs a window.
  - Defined onClose() to close window.
  - Defined onExit() to free window.
  - Changed run() to call onExit() at the end.
  - pollEvents() now polls for sf::Event::Closed to close the window.
  - render() now clears and displays window.
  - Destructor closes and deletes window when necessary.

src/main.cpp
  - Added this basic file to run App for testing.