Add Events#oneTimeListenTo
Add tests for Events#oneTimeListenTo
Rename Events#oneTimeListenTo to Events#listenToOnce
documenting validationError
Merge pull request #2172 from tgriesser/validationError

Documenting validationError
Changed the year from 2012 to 2013.
Merge pull request #2176 from diegotres/patch-1

Changed the year from 2012 to 2013.
Refactor var name `list` -> `events` for clarity, uniformity
Changed the year from 2012 to 2013.
Changed the year from 2012 to 2013.
Merge pull request #2178 from diegotres/master

Changed the year from 2012 to 2013.
Updated documentation for model.changed

* Removed a reference to the obsolete model.change.
* Added (IMHO) missing info about model.hasChanged.
Tweak changed documentation.
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
Update backbone.js

In `Events.once` we can streamline the final statements `this.on(...); return this;` into `return this.on(...)`, as `Events.on` also returns `this`. Saves a row for a very slight loss of readability.

Same thing applies in the `_.each` loop where we set up the IoC versions of `on` and `once`.