implements creation of @2x images for retina displays
Adds option to disable creation off retina images
Adds settings for retina name-label
Updates settings documents with retina options
changes test_a_new to check if log contains saves for retina images
Adjusts tests with new hashes

since _get_thumbnail_filename takes options into the hashing arguments
the test file needed new hashes which supported options with 'retina'
and 'retina_label'
Use THUMBNAIL_ALTERNATIVE_RESOLUTONS + docs.
Fix tests.
Update filtering docs.
Merge branch 'alt_res' into resolutions
image ratio options err.. optional
fix argumenterror in graphicsmagick