Add firstOf computed property

`firstOf` returns the first of supplied properties for which its value is not `Ember.isEmpty`

Example:

```javascript
  var Person = Ember.Object.extend({
    displayName: EmberCPM.Macros.firstOf('nickname', 'name', 'email')
  });

  var person = Person.create({
    nickname: '',
    name: "Jean-Luc",
    email: "jean@starship-enterprise.space"
  });

  person.get('displayName'); // => "Jean-Luc"
```