Support minor keys in Progressions module?

[GoogleCodeExporter]

I'd like to compute e.g. iii in A minor, but mingus.core.progressions doesn't 
recognize 'a' as a key. Using 'A' gives me a chord based on C#.

Original issue reported on code.google.com by da...@iceboundflame.com on 10 Nov 2012 at 8:52