Ember presence

I'm still learning Ember, and coming from Rails, I've found [Object#presence](http://api.rubyonrails.org/classes/Object.html#method-i-presence) really useful.
Searching through the issues and API, I couldn't find anything like this, so I thought it might be nice to have it.

Also, I commented out a spec since I wanted some feedback on that specific case. 
In Rails, if you call `#presence` on an empty Hash , it returns `nil`, but in this case, `Ember.presence({})` returns `{}` instead of null, and `Ember.presence([])` does return `null` (I assume that it might be because `[]` responds to the length method and `{}` doesn't, also I'm using the isPresent method, which uses the isBlank method).


So my question is, is this behavior okay, or should it return null?

Thanks!

BTW I still need to add some documentation to the presence.js file, but I wanted to open the PR to get some feedback