Added pause game music

Pause game music resource, PauseGame.wav, added to project.
Pong screen properly pauses and unpauses

Game now pauses and unpauses with the escape key.  Bonus pause game
music is played while the game is paused.

ScreenManager.IsPaused - To accommodate the managing of the pause state
over multiple screens, a semi-global pause state, IsPaused, has been
added to the ScreenManager class.