Added support for \a (BEL)

User can now choose to activate sounds; if they does so, the BEL character
will trigger a notification sound (include/notification.[mp3|wav]).
Existing sound file is from
  http://soundbible.com/1815-A-Tone.html
and should be public domain.
Merge branch 'master' of https://github.com/xrm/websockify