fix audio_video demo and retrieve prettified code as text to avoid doubl...

Should easyRTC.initMediaSource be called in the connect function of demo_audio_video.js ? Doing so seems to fix the demo, prompting the user for camera/microphone access.
Also, since the JS code to be pretty-printed is now retrieved over ajax, I thought it would be better to set the dataType to text so the code is not evaluated twice.