add support for disabling the icon

It is currently not possible to hide or remove the icon from the ActionBar with the Titanium API. I remember that it was possible to do `actionbar.setIcon(null)` on an earlier version of Titanium but this is not working anymore.

The issue came up in the Q&A yesterday: [remove icon from action bar - android](http://developer.appcelerator.com/question/176632/remove-icon-from-action-bar---android)

Malcolm Hollingsworth has found a workaround by setting a 1px width transparent logo graphic but I think it is more convenient to call `disableIcon()` so I added this function to the actionbarextras.

Version bumped, new zip added, example and readme updated and - of course - tested. :)