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TODO: 
 - allow for processing a directory of files
 - one mode of processing would be to simply copy files to another location
 - one mode of processing would be to rename directories (normalize)
 - one mode of processing would be to look up last.fm metadata
 - one mode of processing would be to look up musicdns puid, metadata
 - one mode of processing would be to set id3 tags based on available information

model the processing modes as having:

 - a Specification for whether they should fire
 - (optionally) an ordering, either linear or DAG, which specified which processes come before which others
 - a routine which runs
 - a marker for success (should cause the Specification to fail)
 - a marker for failure (how will this interact with the Specification)