! routing: add `pathEnd` and `pathEndOrSingleSlash` directive, closes #628

This commit also removes all ambiguity from the `path` and `pathPrefix` directives.
From now on all path directives (except for the new `pathEndOrSingleSlash`) match exactly one single (part of an) request URI.
Merge pull request #649 from spray/ticket/628

+ routing: add `pathEnd` and `pathEndOrSingleSlash` directive