Support providing a list of paths to transcode as a file or stdin

For example:
    find /Volumes/music -type d -name "P*" | ./flac2mp3.py -o /Volumes/mp3-transcode -d /Volumes/music
and:
    ./flac2mp3.py -o /Volumes/mp3-transcode --file transcode-list.txt
and 'interactive' stdin mode:
    ./flac2mp3.py -o /Volumes/mp3-transcode -f - < /dev/stdin

Jobs are now dispatched individually with apply_async for lazy line-by-line
evaluation of the input file, without needing an EOF.  Useful with stdin
as input.
Report errors making directories.  Fixes error reporting regression in previous tmp file changes.

Lame no longer reports output file errors because it outputs to stdout, so we
must handle our own file errors
Show full filename in logs
Fix typo in README.md
Detect and flag errors in FLAC decode

Don't leave failed or imperfect conversions in the output directory

If flac returns an error code, we consider the transcoding to have
failed.  Remove the intermediate file