Restructured data, fixed a bug

[nawroth]

• Flattened the data structure to make it easier to handle (use keys instead of looping over arrays).
• Added a way to get all variations of a chord.
• Aded aliases for root notes (including the real symbols for # and b).
• Added aliases for chord names.
• The label wasn't propagated when a new Chord instance was returned.
• I added the possibility to send in attributes for the label (because I needed a different font there).