Improve ArtifactXXX hierarchy

[arcanefoam]

The ArtifactPlugin, ArtifacterReader, etc.., share some of the itnerface methods. For example all the intefaces define the public abstract String getPluginId();

Perhaps a common super interface could be provided: ArtifactHelper to contain the common functionality. That way, for example, it would be easier to document that the getPluginId(); needs to return the class name of the ArtifactPlugin implementation.

[meik99]

It seems like this method could just be removed and the existing plugins refactored, since the plugin name is already defined when extending the ArtifactPlugin class.