Added support for GALLERY_FOLDERS (replacing GALLERY_PATH in a similar manner as LISTINGS_FOLDERS).
Fixed flake8 warnings (forgot to import sys, and forgot to remove left-over stuff after refactoring).
Made path locator similar to the one in listings.py, i.e. also destination paths can be used.
Merge branch 'gallery_folders' of https://github.com/felixfontein/nikola into felixfontein-gallery_folders

Conflicts:
	CHANGES.txt
print out only proper galleries; sys.exit(1)

Signed-off-by: Chris Warrick <kwpolska@gmail.com>
fix links with or without slashes

Signed-off-by: Chris Warrick <kwpolska@gmail.com>