Refactoring

• Add accessors not to modify instance variables.
• Update docstrings.

Add a utility to find chords from notes

• Support 5th(power) chord.
• Add a utility to find chords from notes.

Implement __repr__

• Implement repr function to print Chord instance on command-line.

Support some functions in ChordProgression class

• Support __add__, __len__ and __getitem__ functions in ChordProgression class.

Add ChordProgression class

• Handle chord progressions with ChordProgression class

Initial release

• Handle more than 30 chord qualities
• Transpose chords
• Get component notes