Avoid gaps that don't fit quantization scheme

[jacobtylerwalls]

Fixes #811

Avoid gaps that don't fit quantization scheme (1/6 QL rests, etc.) by looking ahead to see the divisor that will be used to quantize the subsequent offset.

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Example file: https://github.com/cuthbertLab/music21/blob/master/music21/midi/testPrimitive/test01.mid

Finale (reference)

m21 roundtrip Before

m21 roundtrip After

[coveralls]

Coverage increased (+0.01%) to 92.356% when pulling 023dae9 on jacobtylerwalls:quantize-gaps into 5d566a8 on cuthbertLab:master.

[mscuthbert]

Really great. Just some comments about existing bugs and namedtuple for style.

[mscuthbert]

Thanks! Looks great!