Add Events#oneTimeListenTo
Add tests for Events#oneTimeListenTo
Rename Events#oneTimeListenTo to Events#listenToOnce
Return "this" reference

Return "this" is a good convention that allow to use chains.
Merge pull request #2186 from DreamTheater/patch-2

Return "this" reference
Return "this" reference

Forgot one more return statement.
Merge pull request #2187 from DreamTheater/patch-3

Return "this" reference
Merge pull request #2049 from gsamokovarov/listen-to-once

Add Events#listenToOnce
tweaking variable name