Add support for label width property as per the iOS version.

The titanium iOS version of tabbed bar supports a width property for labels but this one doesn't.

This commit adds support for that label width.

If you prefer, more effort could be put into the case where a developer only specifies a width on some, but not all, labels of a tabbed bar, but for now I have made it so you really need to have widths for all or none.

See http://docs.appcelerator.com/titanium/3.0/#!/api/BarItemType and the labels property at http://docs.appcelerator.com/titanium/3.0/#!/api/Titanium.UI.iOS.TabbedBar