Add function for testing load path of 2x image
Remove #is_external

We don't need this anymore because we're going to try to load the image
regardless of where it lives.
Check 2x variant by attempting to load 2x image
Fix condition for checking confirmed 2x paths
It's safe to remove the config now, because it's no longer used.
Merge commit '93db8c1f82bc4661668c618fff5931ea12033cb4'

Conflicts:
	src/retina.js
	test/retina_image_path.test.js