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) and [metal apply](https://github.com/megawac/metal.js/commit/bd0e20db019fd05fc8508480ff28253fa848e44c)

This also will resolve https://github.com/jashkenas/underscore/pull/1965 and fix https://github.com/jashkenas/underscore/issues/1883

(Related discussion from @jdalton https://github.com/codemix/fast.js/pull/50)

(sorry about the branch messed up my remotes)