Fixed event listeners and playbacks.

Before, the 'ChannelLeftBridge' event listener was being registered
multiple times. These changes fixed the issue by only registering
the event one time for each bridge. Now, the playbacks occur at the
desired time, instead of happening in incrementing amounts.

* Relocated event listener to only register once
* Playbacks occur reliably now, and as expected
Merge pull request #3 from bkford/master

Before, the 'ChannelLeftBridge' event listener was being registered
multiple times. These changes fixed the issue by only registering
the event one time for each bridge. Now, the playbacks occur at the
desired time, instead of happening in incrementing amounts.

Relocated event listener to only register once
Playbacks occur reliably now, and as expected