Renamed data-gwt-image to data-gwt-image-resource.

This will make the connetion between ImageResource and the directive
a bit more explicit. Also starting to work on TextResource and
DataResource.

https://github.com/kyoken74/gwt-angular/issues/57
Added StringTextResource for convenience. Working on gwt-data-resource.

Still working on the directives for gwt resources. This is an
intermediate commit.

https://github.com/kyoken74/gwt-angular/issues/57