Add firstOf computed property

firstOf returns the first of supplied properties for which its value is not Ember.isEmpty
firstOf -> firstPresent
Use isBlank instead of isEmpty

See [`isBlank`](http://emberjs.com/api/#method_isBlank) vs [`isEmpty`](http://emberjs.com/api/#method_isEmpty).
Wrap Enumerable method with Ember.A
Use isBlank where possible
Backport isBlank if not available from ember
Merge pull request #36 from amiel/add-firstOf

Add firstPresent computed property