update model to safely reject binding attempts to disconnect once already destroyed

Enyo-DCO-1.1-Signed-off-by: Cole Davis <cole.davis@lge.com>
Merge pull request #490 from enyojs/model-update

MERGE https://github.com/enyojs/enyo/pull/489 FIRST

Reviewed-By: Ben Combee (ben.combee@lge.com)