Implement pushtotalk

I took a crack at the request here, http://code.google.com/p/bigbluebutton/issues/detail?id=1292 .

To enable it you can add the line, pushToTalk="true", to the ListenersModule entry in your config.xml.

When set to true all new listeners start muted and you can either hold down the Mute/Unmute button or hold down "ctrl+shift+M" to talk.

There's also some commits with fixes for accessibility stuff that I didn't know how to revert.