allow validators to be asynchronous

Currently the implementation of `Ember.Validations.validators.Base` does not allow validator subclasses to be asynchronous. This can be changed quite easily just by casting the result of `validator.call()` to a promise.


**Note**: the code for the cast is cribbed from https://github.com/emberjs/ember.js/commit/e41fd7573c2dab5d6eba59763adb4c16f3957820
and can be replaced with `Ember.RSVP.Promise.cast(this.call());` when that version of ember is released.

Current tests pass with this patch, and if you are amenable to accepting it I can write a test for an actual asynchronous validator.