[FEATURE isPresent] Add Ember.isPresent

Maybe it's because I come from rails, but I miss having `.present?` as the inverse of `.blank?`. I find that I end up using `! Ember.isBlank()` more than `Ember.isBlank()` by itself. I know this is a simple change, but I think the concept of presence is useful and more semantic.
@pangratz Thanks, I've added a test.