Slightly more robust port checking

Sup, I kind of added a function called valid-port-p which checks if there's a valid port in an URL. It's slightly lacking, 'cuz it'll let http://url.com/:8080:70 pass, but at least it'll handle stuff like http://url.com/:some-silly-stuff/:8080 correctly, which I don't think that your code did before.

Anyway, just leave a comment if you think that something is severely lacking or anything, I'm gonna go have me some dinner.