add prefered/default protocol in the sites framework

[Eric]

I think adding a prefered/default protocol in the sites framework would be a great addition for generating full url's to django websites.

specially when generating url from action not coming from a request like the sitemaps ​ping_google that assumes the protocol to be http:

    current_site = Site.objects.get_current()
    url = "http://%s%s" % (current_site.domain, sitemap_url)

so we could have something like this instead:

    current_site = Site.objects.get_current()
    url = "%s://%s%s" % (current_site.protocol, current_site.domain, sitemap_url)

and maybe even add a shortcut for the site's base url

[Eric]

proposed changes