bridge-infinite-wait/example.py: Re-tool Python example

This updates the Python example for bridge-infinite-wait in the following ways:
(1) It adds an announcement that occurs periodically
(2) It removes one of the channel left bridge handlers. The only time we need
    to handle this is from the bridge itself.
(3) We now toggle MoH on/off the bridge surrounding the announcements. This is
    to work around a limitation in Asterisk, where we don't currently pause
    and restart MoH when an announcement is triggered.
bridge-infinite-wait: Update JavaScript application; tweak Python one

This updates the JavaScript application to periodically playback a message
letting everyone know in the holding bridge that we thank them for their
patience. Insulting, yes; informative, no.
example.js: Fix syntax errors; call ChannelLeftBridge callback on channels

In addition to fixing some egregious and obvious syntax issues, this patch
also makes it so that the ChannelLeftBridge handler is called on a channel
leaving the bridge, as opposed to on the bridge itself. This allows for
safe destruction of the bridge when the channel count reaches 0. Previously,
the callback was called for each channel in the bridge when a channel left,
which would cause the application to attempt a bridge.destroy() more than
once.
example.js: Use .once instead of .on for events that should only occur once

This includes the PlaybackFinished event for the periodic announcement
messages, as well as the ChannelLeftBridge event (as we only expect them
to be leaving the bridge one time).
Merge pull request #2 from matt-jordan/infinite-wait

bridge-infinite-wait: Update examples to have a periodic playback