expose iconName and iconsetName as read-only properties (for testing)

[mgiuffrida]

Because of tools like poly-icon.appspot.com, an iron-icon's icon name may change when an app creates its own iconsets. A test that checks icon.name == 'icons:clear' would fail if icon.name becomes 'my-app:clear'.

It would be cool if the icon name (the part after the :) were a gettable property, so tests could verify the icon name without being dependent on whatever the iconset name is.