Added #num_values method to PitchClass
Added notes on beat detection approach
getting rid of unused best_match_key method
Added 'chord' class. Added musical key estimation.
Added notes on the key estimation implementation.
Added guard to make sure callers pass in a PitchClass, not a Pitch
Specified min version of 'hmm'
Added optional parameter to set analysis hash when creating events and notes.
WIP commit. Working on generating a better end-of-phrase-boundary detector.