Move JSObject and friends to gwt-util

[kengu]

I'ts time for some restructuring and refactoring before first official release. This one should not affect the API, so no breakage is expected. But, anyone using JSObject in project dependent on gwt-leaflet, will break.

These are the changes:

1. Update JSObject and friends to latest gwt-openlayers version
2. Move JSObject and friends to org.discotools.gwt.util.client in gwt-util
3. Add missing utility classes - Attributes, ElementHelper and data type wrappers.