Fix with horrible hack

Fixes #179 with a hack, could do with a better way of doing it.

Another way would be: make the deserialize function in attr.js public, e.g. Ember.Model.deserialize, that way it can be called from revert method in model. But this breaks encapsulation.

The best way would be if there was a way to invalidate the attr property in revert, e.g. `this.invalidateProperty(attr)`, but I don't think that exists in ember. Could fake it with a made up dependant key on each attr? But that would be equally hacky. @ebryn, any idea how to do this in a non hacky way?