Add _.apply method

`_.apply` calls some callback in an optimized way, useful in places such as [backbone.triggerEvents](https://github.com/jashkenas/backbone/blob/master/backbone.js#L247-253).

This also will resolve https://github.com/jashkenas/underscore/pull/1965 and fix https://github.com/jashkenas/underscore/issues/1883