Add support for coercing valid URLs

[jcf]

This makes it possible to coerce a URL string into a java.net.URL.

The decision to use a URL as opposed to a URI comes from the fact we want to ensure there is a protocol in the URL string.

We can add support for both via uri and url functions if anyone is interested in the differences between the underlying classes we coerce to.

[weavejester]

We probably want to do this via a coercion, i.e. something like:

(defn string->url [_ value]
  (if (valid? schemes value)
    {:value (URL. value)}
    {:errors #{(invalid-url schemes value)}})))

(def url-coercions
  {[String URL] string->url})

This will allow the writing of rules like:

{:website URL}

That are independent of input. In the case of JSON we'd accept a string, but with edn we might have something more sophisticated.

[jcf]

That's very nice! We should advertise this functionality in the constraint wiki.