switch back to urllib2 to add headers, correct name format

HTTP header "Host":"namecoin.dns:80" disambiguates which service is
being requested when multiple hosts are served from a single IP address.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html (section 14.23)
Note explicit inclusion of port 80, may or may not be a good idea in
this context (feedback appreciated).

names are now expected to begin with "d/" or "id/"