Add enyo.bindSafely method, refactored from enyo.Object.bindSafely

Update enyo.Model and enyo.Collection to use enyo.bindSafely for callbacks

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #427 from enyojs/refactor-bindSafely

Refactor enyo.bindSafely