using Note - Note to calculate intervals instead of Note.value

Dear Pedro,

I noticed the interval between B' and C'' was -11. I was expecting 1 as answer, giving the ascending octaves of the notes. I also noticed that Note.__sub__ gave the expected answer. Just to illustrate:

    from pyknon.music import Note
    c6 = Note("C''")
    b5 = Note("B'")
    c6 - b5
    Out[1]: 1
    NoteSeq("B' C''").intervals()
    Out[2]: [-11]

In this way, I just changed NoteSeq.intervals() making it use Note.self instead of Note.value. It seems to fix that. What do you think? I also added a proper test.

Thank you my friend!