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`.
added event functions chaining tests
also tested noevents and eventsAPI code branches
Tweak chainable events tests