Defined Game::quit() function.

- Created quit() function to set the running boolean to false, making it
  possible from outside the class and thus from other states on its
  StateManager's stack.
Finished Sound class.

- The Sound class is now complete. It supports wma file loading for
  sound effects.
Shortcut function for GameState, alignment functions for Window.

- Added a function to the GameState class to act as a shortcut
  invocation of the StateManager::popState() function.
  
- Added a multitude of functions to help center Blittables to specified
  x- coordinates, y-coordinates or both x- and y- coordinates. Also
  added some to align a Blittable with the top, bottom, left or right
  sides of the window.