Make Accidental non-optional

[robtimp]

Addresses issue #76

I believe make the Accidental type non-optional for Tone and KeySignature will help prevent some odd behavior and make for a more consistent model.

I've also reordered the initializer for Tone to place accidental after note letter, which better matches how we natural refer to the notes (i.e. G-sharp, not sharp-G).

[robtimp]

@drumnkyle @migue48 I feel like the question of whether to display a natural before a note is one of presentation rather than semantics.

Thoughts?

[codecov-io]

Current coverage is 86.87% (diff: 100%)

Merging #82 into master will decrease coverage by 0.12%

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