`on` is a noop if no callback is provided

…pe.on and Events.prototype.once

Rationale:

  - As stated by @jashkenas in jashkenas#1008 binding a event without passing a callback is "an error by definition".

  - "Errors should never pass silently".

  - If I reference a method that do not exists in a view `events` hash,
    it will throw and error when I instanciate the view, and I'll fix that issue in a minute.

  - Before jashkenas#1008 a typo like `.on('change', this.rander, this)` would have thrown an error quickly.
    Now since 0.9.9 these typos became a noop, so the only way to spot this kind of issues is to wonder
    why your callback is never called.

I know this is a backward incompatible change, but it will only break dead code writen for 0.9.9 or 0.9.10.