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
Merge pull request #974 from KraYmer/fetchart_issue848_2

embedart: write candidate image only if it is "similar" to already embedded one (suite)
Minor fixes, changelog for #974