Publish the defaultZ property.

When using the Onyx library with another UI system like Bootstrap3, I found it useful to be able to adjust the defaultZ value to be greater than some floating elements on my page.
Update Popup.js

Comment typo fix
Fixed mix of tabs and spaces.
Merge pull request #198 from GlitchTech/patch-1

Publish the defaultZ property.

Reviewed-By: Ben Combee (ben.combee@lge.com)