lastfmcovers plugin should not fetch "generic" cover

[Matteo Beniamino]

Exaile from http://bazaar.launchpad.net/%7Eexaile-devel/exaile/exaile-0.3.0/ at revision r2391 .

If an album exists in the lastfm DB but the actual cover is missing, lastfm shows a generic picture. The plugin imports this generic picture as the album cover in Exaile. I think that the expected behaviour should be to ignore this generic cover.
To check the current behavior assign "Wolfgang Amadeus Mozart" as the author and "Mozart Symphonies 10" as the album to a custom track and fetch the cover with lastfmcovers as the default plugin.

The proposed attached patch introduces a (black)list of sha1 signatures of pictures to be ignored.

[reacocard (reacocard)]

patch committed in r2396, thanks!