Minor bug fixes, added comments + unit tests

- Added basic unit tests, + doc-tests to harmonies
- Added some comments/questions
- Fixed some bugs in harmonies
(P1 not found, since it had been named interval_P1)
(curry is not a python function; at least not in python 2.7/3.0)
- Changed chord function name from chord_* to *_chord
I think it sounds better to call 'power_chord' than "chord_power" ;-)