Application is no longer an interface class and has been removed from
the engine. It is now considered part of the game itself.

Some functionality tweaks to GameStateManager.

Modified Application to use GameStateManager in the run loop.
NOTE: Also updated the Application::poll() method to clear the
GameStateManager of all states which will cause Application::run() to
simply end.

Removed the SDLApp class from main.cpp and modified the main() function
to create the Application class directly.
NOTE: Currently, no state is added to the Application. This program will
now simply close immediately when executed.

TODO: Ummm... make an actual State! Wheee!