Fixed a menu mousedown event bug and added a crude About page.

- Fixed a bug where if you clicked anywhere on the menu that didn't take you to another menu state, gameState would be set to STATE_SIMON_PUSH instead of simply remaining in STATE_MENU until a valid option was clicked.
- Added the menuState MENU_PLAY and differentiated it from MENU_NONE, which was encompassing it's responsibilities and causing the bug.