Add second parameter to duration & make tests pass

Hello,

I've implemented the duration helper/computed property, additionally adding support for a second parameter (as supported by moment.duration()). Example would be like so:

```hbs
{{duration 1 'hours'}}
{{! outputs 'an hour'}}
```

You can also pass first parameters other than milliseconds (such as an object), as defined in the moment docs:
http://momentjs.com/docs/#/durations/creating/

Let me know what you think! Hopefully this gets pulled into the source repo.