artresizer: add check_method()

for the case where we want to know if a specific method is available
add stderr to exception raised by command_output
embedart: add compare_threshold option

if compare_threshold > 0 we call check_art_similarity to return sooner
if it happens that candidate image and embedded one are similar.
embedart: add tests for 'compare_threshold' option

was tricky to write meaningful tests when computing phash of existing 6
pixels image (image-2x3.jpg) so I added new -more representatives-
images.
embedart: docs. explain 'compare_threshold' option
obey Travis orders
reduce image sizes
check_method return method version, add _can_compare()
add can_compare flag to ArtResizer
skip art compare tests if no recent IM version
fix flake8 warnings
Merge branch 'fetchart_issue848'

Conflicts:
	test/test_embedart.py
fix flake8
artresizer: parse output of `identify` command to check IM version

we used to parse `convert` output but `convert` happens to be a
Windows cli command too. using `identify` is less error prone.
restore command_output() implementation as of 0ec285f
lyrics.py: fix regexes used by strip_cruft (make them case insensitive)

strip_cruft() should now correctly replace all <br> with \n thus making
insert_line_feeds() and sanitize_lyrics() functions superfluous (they have been
removed).
lyrics_sources: put sources in their correct category
test_lyrics.py: improve lyrics module coverage
remove a log.info()
Merge pull request #977 from KraYmer/lyrics_test

Lyrics test