use bpm restrictions in putting together songs
-- goodness function built around mixes with low stdev of bpm
-- only add songs within bpm range +/- 5 bpm
add ability to shift tempo to match some standard in the mix
-- note that this may require some logic like in (1) due to the desire to keep songs close in tempo to avoid artifacts
-- not sure about what range is acceptable here.
tempo or beat matching (see: http://www.surina.net/soundtouch/)
need to think about how to control this (from seed song?)