Streamlined offsetTree.py.
Renamed OffsetTree to TimespanStream.

This datastructure holds timespans, and it's more important to describe the
structure in terms of its purpose (a "stream" of timespans") rather than in
terms of its implementation (a specific kind of tree which keeps track of
offsets).
fixed docs in clef and note
Merge eidanch patch for Features
Extending TimespanStream to support __getitem__ and __getslice__.
Preparing TimespanStream for __getitem__ logic.
First commit on personal branch.
Finally got everything set up.
Finished tests and docs for naturalLanguageObjects
Fixed setup on desktop system
Fixed setup on surface
Update .gitignore_global

add __pycache__
Implemented node-indexing logic in TimespanStream.
Renamed module music21.stream.timespanStream to music21.stream.timespans.
Merge branch 'master' of https://github.com/cuthbertLab/music21
Renamed timespans.TimespanStream to timespans.TimespanCollection.
Implemented __getitem__ logic on TimespanCollection.
Improved TimespanCollection docstrings.
Streamlined TimespanCollection.__getitem__() logic.
Streamlined TimespanCollection.__getitem__().
Implemented __setitem__ on TimepanCollection.
Merge pull request #57 from davidvperez/master

NaturalLanguageObjects -- pitch from string in foreign language