convert_music: Done with ConvertFiles.convert.

Finished method that does the actual converting from FLAC to mp3 with a
wav file in between.

Before I just piped the output from flac to lame, but this required
subprocess.Popen(shell=True) which is dangerous. I decided to go with
shell=False instead, which doesn't allow piping as far as I know.

Added more items to the Queue.Queue() tuple with all the necessary
filepaths.

Will put all tag-related processing in .write_tags(), so .finalize() was
just gonna rename. Got rid of that and will just use os.rename() in
.run() instead.

Testing for debug messages in .convert() required me to change
conftest.py to enable 'verbose' keyword argument.