Return `this` from `#listenTo`
Register stopListening once instead of wrapping
Remove duplicated context

We’re binding, no need to add a context to it.
Test both `_listeningTo` and `_events` are empty
Clean up references on both listener and listened objects
Invert the for loop
Merge pull request #22 from jridgewell/listentoonce-is-borked

Register stopListening once instead of wrapping