PICARD-418: fallback to using release group cover

It adds the option to use release group cover art from CAA.
It is required to enable CAA and release group fallback is disabled by default
Images can have other types (png, jpg, gif), remove '.jpg'
Fallback -> Fall back
Move fallback option to cover archive options group
Honour CAA image size preferences (full/250/500)
Remove useless line after acab1dc
Be sure to continue on next provider if no front image is found.
Make CoverArtProviderCaaReleaseGroup inherits from CoverArtProviderCaa

It solved many issues, re-use json parsing code, and handling of various caa options (types, sizes, etc..)
Do not hide exceptions from cover art providers
_caa_path() -> property
Ignore 404 errors for release group json

We currently have no easy mean to know if there is cover art for the release group as we do for the release.

Not a perfect solution, ideas and patches are welcome.
We should not query the json file at all if we know there is no cover art for this release group.
Update NEWS.txt
Reduce code redundancy between classes
Inherit CaaCoverArtImage and CaaThumbnailCoverArtImage

It eases debugging using classes named CaaCoverArtImageRg and CaaThumbnailCoverArtImageRg for release group fallback

Example of log line after this change:
D: 17:08:23 Cover art image downloaded: CaaCoverArtImageRg(url=u'http://coverartarchive.org/release/06c97185-65df-4c2b-b803-428c710bedc0/2153324479-500.jpg', types=[u'front'], is_front=True) [w=352 h=350 mime=image/jpeg ext=.jpg datalen=35765 file=/tmp/picardrdYsnE.jpg]
Replace numeric error code by more explicit constant ContentNotFoundError
Merge pull request #348 from zas/rg_coverart

PICARD-418: fallback to using release group cover