This turns on metadata-writing based on the import.write config option, so
those with this option turned off will be spared any surprises. (Affects #217
and #143.)
This is accomplished via a new event, "import_task_apply", which is called
right after metadata is applied to newly-imported items.
This change makes chroma REQUIRE a new version (0.6) of pyacoustid. Users with
older versions installed will see complaints about a missing method
"fingerprint_file".
