Add Events#listenToOnce

As per discussion in #2045, I implemented `oneTimeListenTo`, an inversion-of-control version of `once`.  @caseywebdev and @tgriesser voted for the name `listenToOnce`, but I went with `oneTimeListenTo` as it sounds clearer to me, but its up for discussion. If you guys like `listenToOnce` or some other name, I can change it.

Edit:

Changed the name from `oneTimeListenTo` to `onceListenTo`. Its still up for discussion, though.

Edit:

Changed to `listenToOnce` as per the comments.