+ routing: add parameterless `/` modifier for PathMatcher

So you can say `path("foo" /)` instead of `path("foo" ~ Slash)`.
= httpx: fix TwirlSupport XML marshalling

When the client is a browser the `twirl.api.Xml` marshaller was always rendering to `text/html` instead of `text/xml` (since we had added support for q-values in the meantime and browser prefer HTML over XML).
Also see #664 for a related feature ticket.
= site: fix broken routes, add missing reveal.js plugins for scala.io slides
Merge pull request #665 from spray/wip/misc

+ routing: add parameterless `/` modifier for PathMatcher, other smaller things