Artifact class should support 'name' or 'path' method

I wanted to copy an artifact to a specific directory, but couldn't find any way to determine the filespec to pass to cp. Apparently to_s works, but it would be better to have an obvious method to obtain the full filespec (path+name). Honestly, it seems like this belongs in FileTask, so it would be available to any type of file.

Also, whatever is chosen should be documented. Even if it just stays as to_s, that should be clearly mentioned in the api docs, and in the user guide.