More file and URI stuff

I think it's good to share some ideas through code. For a start I did some cleanup on the wiki code base.

Reason is that what annoyed me is that the SymbolicLinkResponder does basically the same checks that the (former) SymbolicLinkFactory did. ... but instead of working around that I moved the dependencies from FileSystemPage to ExternalSuitePage and SymbolicPage out of the (base-) wiki page code into a SubWikiPageFactory.

I also noticed that the current "relative" links are relative to the current working directory, not relative to the fitnesse root path. Having a fallback for relative links would be nice, as I expect it to be the most common used feature.