Move redundant code to its own method _download()
Pass url since relations are all of url type.

It also fixes a minor potential issue, by stopping loop if a special site url was found
and processed.
In case of cdbaby url (ie from http://musicbrainz.org/release/1243cc17-b9f7-48bd-a536-b10d2013c938),
next test was matching and was adding http://www.cdbaby.com/cd/vasb/from/musicbrainz
as image url to try_list, though no download attempt was occuring because real
image url was processed just before and next one was removed from list (in _coverart_downloaded()).
_process_url_relation() renamed _process_special_sites()
_process_asin_relation() renamed _process_amazon_url()
_fill_try_list() renamed _process_release_relations()

Because it is what it does.
_walk_try_list() renamed _download_next_image()

More explicit.
_caa_append_image_to_trylist() renamed _try_list_append_caa_image()

More consistent with _try_list_append_image() used directly for other providers
Minor indentation fix
self.tagger.window.set_statusbar_message -> self.display
Move CAA url template outside method run()