self.get_urldata() is eqivalent to get_http_data(self.url), but
also caches the data, so no additional requests are made if it
is called multiple times (e.g when grabbing title or downloading
thumbnail).
Generic().get(url) still causes it to be fetched an extra time.
The default handle method will look for a supported_domains attribute
(a list), containing the supported domains. The subclassed service
class can of course override this if other means of determining support
is needded.
