Bug fix: unsubscribe hierarchy

This release has a minor bug fix to unsubscribing hierarchies.

From #97

PubSub.subscribe('a', myFunc1);
PubSub.subscribe('a.b', myFunc2);
PubSub.subscribe('a.b.c', myFunc3);

PubSub.unsubscribe('a.b');
// no further notications for 'a.b' and 'a.b.c' topics
// notifications for 'a' will still get published

Thanks to @jbutz for the investigation and the fix