Restructured data, fixed a bug

* 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).