Site model adjustments

@gasman, for the record (I've lost your original comment, probably as I rebased) it's stated at http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.23 that we can expect `HTTP_HOST` to have an optional `:portnumber` part. Still, as you mention that it's there in `request.META['SERVER_PORT']` (and seems to be even on runserver), I'll rewrite to use that. Should avoid needing to catch the ValueError.