Listeners get logging level modulations, like import_stages already did.
Add extensive tests:
- explicit plugin commands are logged like core beets (INFO or DEBUG)
- import stages are more silent
- event listeners are like import stages
Delete @BeetsPlugin.listen decorator since listeners need plugin instance
context to set its logging level.
Improve #1244. Next is multiple verbosity levels.
Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
