local: Add local_to_file_uri translator
local/scanner: Make checking mtime and skipping known files work again

This change just patches over the worst of the inconsistencies in how the
scanner mixes local and file uris to get us to a working state again. Ideally,
this still needs a real cleanup when we finish the plugable library providers
and/or json library work.
local: flake8 fixes
Merge pull request #567 from adamcik/feature/local-uri-translation

local/scanner: Make checking mtime and skipping known files work again
docs: Update changelog for PR #566 and #567
Merge branch 'develop' into feature/subcommands

Conflicts:
	mopidy/scanner.py