Support "file:" links with no authority specified (Issue #244)

Pull request for the patch supplied in Issue #244:

* `src/fitnesse/wiki/fs/FileSystemPageFactory.java`,  `src/fitnesse/responders/editing/SymbolicLinkResponder.java`

  Main change.

* `src/fitnesse/wiki/fs/FileSystemPage.java`

  Multi-platform correctness, also necessary for passing the tests under Windows.

* `FitNesseRoot/FitNesse/UserGuide/FitNesseWiki/SymbolicLinks/content.txt`

  Minor documentation update.

* The rest are updates to the tests.