The `AnnotationSync` class was an ES5-style class that had a ton of
ES5-isms, odd constructions probably resulting from a historical
conversion from CoffeeScript and unnecessary abstractions. It was also
lacking in documentation.

This commit rewrites the class using modern syntax and removing unused
code and unnecessary abstractions (eg. the `_eventListeners` and
`_channelListeners` properties).

The API and behavior should be unchanged. An additional test was added
for the `sync` method.