WIP #16543 add directives to add or remove trailing slash

Added two directives as said in #16543.

Need to discuss:
- The location of the directives: `PathDirectives` or `RouteDirectives`?
- Returning status code to redirect: 301, 302 or make it configurable? 
- More tests
- More documentation
- etc.
RFC 7230 mandates that URIs need to be normalized before being evaluated (http://tools.ietf.org/html/rfc7230#section-2.7.3), which is actually a somewhat intricate process (http://tools.ietf.org/html/rfc3986#section-6). Since this is easy to forget we leave the user no option here, it is simply the "right thing to do". Also, by folding the information from the `Host` header and the encryption state into the URI we give the user always an absolute URI, with the right scheme, the right host (authority) and full, normalized path to work with.

I think the cases where you really need to look at the raw URI as it came across the wire are rare.
Still, you can do it by enabling and inspecting the `Raw-Request-URI` header. But we want to make this sufficiently difficult so you really know what you are doing when going down that route.