add enyo.EmptyBinding as an alternative to enyo.BooleanBinding, needed to fix bug with enyo.GridListImageItem

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #448 from enyojs/add-emptybinding

add enyo.EmptyBinding as an alternative to enyo.BooleanBinding, needed t...

Reviewed-By: Ben Combee (ben.combee@lge.com)