join - composeable CPM support



````javascript
function upcase (dependantKey) {
  return Ember.computed(dependantKey, function () {
    return this.get(dependantKey).toUpperCase();
  });
}

var Obj = Ember.Object.extend({
  firstName: 'Jean-Luc',
  lastName:  'Picard',
  formattedName: join('firstName', upcase('lastName'), ' ') // 'Jean-Luc PICARD'
});
````
Do you feel we should be supporting using a property for the separator string?

```js
var Typ = Ember.Object.extend({
   sep: ', ',
   firstName: 'Mike',
   lastName: 'North',
   listString: join('lastName', 'firstName', 'sep')
});
```