"advanced" guide on manually choosing path formats
fix #508: crash when parsing empty query part

This is a regression created by my endeavor to have parse_query_part never
return None. Now the regular expression should really always match.
DBCore types: no functional-style Type constructor

This was getting more and more awkward. Also added a `parse` method (in
progress).
less code to specify `sql` and `query` for Type

Declarative class members instead of super calls. Wins every time.
avoid NotImplementedError for the time being
Search again locally in album folder when running beet fetchart without -f
Colorized output of beet fetchart, for easier detection of albums with no cover
comments, naming, and docs for part of #509
merge colorization part of #509

Conflicts:
	beetsplug/fetchart.py
fetchart colors (#508): simplify, new colors

Also a changelog entry.
Refactor import test
Extract ImportHelper for tests
Extract _run_import and expose config in tests
Use _setup_library() helper
Extract library file assertions
Make test strings more verbose
So this is how python works! ;)
Move prune and file tasks
These tests are already in ImportNonAutotaggedTest
Stupid typo