Ember.computed.notNone

This is simply a counterpart of `Ember.computed.none`, in line with `Ember.computed.empty` having an `Ember.computed.notEmpty` counterpart.

On a side note, it is rather cumbersome and inconsistent to declare an inverse for these computed properties. Do we declare more inverses for `equal`? Or `bool`? IMO the canonical way of writing these should be unified, and should probably go through the `not` helper. One idea is to allow chaining of computed properties helpers, such that devs can prepend `not` in front of other helpers - much like in programming languages.