! routing: redefine `PathMatchers.Empty` as `PathMatchers.Neutral` with explicit type annotation, fixes #339

`Empty` is not a good name for a PathMatcher that always matches without consuming anything, as it can easily be misunderstood as a PathMatcher for `Path.empty`. We therefore rename it here to `Neutral`.
Merge pull request #357 from spray/ticket/339

! routing: redefine `PathMatchers.Empty` as `PathMatchers.Neutral` with explicit type annotation, fixes #339