echo nest: top-level code review pass
echo nest: top-level code review pass

General Python/beets style changes:
- "x is None" can generally be replaced with "not x"
- prefer iteration over repetitive lines
- config[foo].get(bool) can be replaced with config[foo] in conditions
- removed extraneous "return None", which is what Python functions do
  automatically if you don't return anything
- 79-column wrap

google-code-export/beets,google-code-export/beets,google-code-export/beets