Incorporate MedleyDB Stems

[ejhumphrey]

Medley has solo stems that could be diced up and incorporated into this collection

• http://medleydb.weebly.com/
• https://github.com/marl/medleydb

[ejhumphrey]

This might not really work, since many of the "instruments" are actually sections -- should confer with @rabitt

in any event, the taxonomy is here, might be worth harmonizing the efforts