slightly less hacky way to prefer dl.fp.o for TC/RC releases
    
    This is still kinda ugly, but none of the six different ways I
    came up with was really *pretty*. Such is life. This allows
    images found by TC/RC Release objects to have self.url set
    to dl.fp.o/foo instead of download.fp.o/foo, and also does the
    same for the https_generic_url for the TC/RC release class.
    It also adds specific URL properties for Image objects when
    a mirror path is available: you can get the direct_url, the
    mirror_url, and the rsync_url. Which might be useful sometime.