Instructions + Sound for main menu buttons

Added instructions screen to game. Exact same mechanics as the credits screen. Checked for editable/selectable (you can't). Also added sound to the buttons: menu to credits, menu to instructions, and all back to main menu buttons. 

Note: If you push 2 buttons really fast in a row the 2nd click doesn't make a sound. Not sure why- maybe the sound file is too long (with silence padding) but I am not very concerned about this.