Use enyo.EmptyBinding instead of BooleanBinding for mapping content to showing

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #54 from enyojs/add-emptybinding

Use enyo.EmptyBinding instead of BooleanBinding for mapping content to s...

Reviewed-By: Ben Combee (ben.combee@lge.com)