Add `Ember.computed.join` shorthand method.

`Ember.computed.join` joins the result of each dependent property into a
single string, with the supplied separator.  This is analogous to
JavaScript's Array.join, which is used inside the implementation.

Example:

```javascript
person = Ember.Object.create({
  firstName: 'Jean-Luc',
  lastName:  'Picard',
  fullName:  Ember.computed.join(' ', 'firstName', 'lastName')
});
```