[Enhancement] Adding moment computed fixes #6

This PR is dependent on #9.  Will squash once you thumbs up and merge #9 .
I would love to see this as a __settable__ computed property macro, which respects the type and format of the non-moment date-ish thing it's bound to.

Example:

```js
var MyType = Ember.Object.extend({
  prettyDate: formatDate('dateAsString', 'MM/DD/YY'), // "10/03/2014",
});

var obj = MyType.crate({
 dateAsString: '2014-10-03T19:30:02Z' // ISO 8601 string
});

myObj.set('prettyDate', '10/24/2014');

myObj.get('dateAsString'); // '2014-10-24T00:00:00Z'   <-- Updated to 10/24
```