add Core#_createTouchToStartScene
display the string using Label in TouchToStartScene
add enchant.ENV.USE_TOUCH_TO_START_SCENE
change condition for display TouchToStartScene
play silent sound in TouchToStartScene if use webaudio
Merge branch 'touch-to-start' into develop

Conflicts:
	dev/src/Env.js
add jsdoc comment for enchant.ENV.USE_TOUCH_TO_START_SCENE