add _.results() method and test

When working on project with Backbone, I often find such a pattern that I need to define a config object inside View/Model/Collection, which contains both value and function. Something like this

```javascript
var Foo = Backbone.collection.extend({
  this.query: {
    perPage: 10,
    page: function(){this.nextPage()};
  }
});
```

I have not found a quick way to convert `this.query` to object with plain values only, when `_.result()` only deals with single key. So I added this method `_.results` with will iterate a whole one level only object, invoke each function and return values, as well as applying context as needed.

Hope this would be useful.