Improved resource copying

Hi Eric,

I'm proposing another change to the resource copying. After my last pull request, if there's some other library on the test classpath whose author was "optimistic" enough to put a "css" or "images" package in the root package (which, incidentally, one of our own libraries does…), this may (or may not, depending on the order of libs in the classpath) override the one in the specs2 JAR and cause the wrong or no resources to be copied to the HTML directory.

This change goes to somewhat greater lenghts to get the resources from the correct JAR - somewhat heuristically. I've manually tested (aka "tried" *<:o) ) it with various combinations of JARs. I'm not sure if it's worthwhile to try to automate a test for this. 

Hope this helps.

Kind regards
Andreas