Add support for the label image attribute as per the iOS version.

This commit makes the label image attribute work as per the iOS one.

See http://docs.appcelerator.com/titanium/3.0/#!/api/BarItemType or the labels property at http://docs.appcelerator.com/titanium/3.0/#!/api/Titanium.UI.iOS.TabbedBar

If you prefer to to have multiple ternary operators on a single line, as used for title I can change it.