Moved offsetTree module from analysis into stream.
Finished migrating offsetTree into stream.
Renamed ElementTimespan.duration to ElementTimespan.quarterLength.
Updated OffsetTree TODOs.
added quantizePot option to midiFileToStream
Cleaned up stream/offsetTree.py.
Merge branch 'master' of https://github.com/cuthbertLab/music21
rem extractMeasureAndMeasureOffsets
Refactored OffsetTree score-creation logic.
Improved OffsetTree docs.
Improved OffsetTree docs.
added _DOC_ORDER for offsetTree
Update music21 so it (almost) runs on Python3 in addition to Python2