Improved docstrings and doctests of the metadata subsystem.


git-svn-id: http://music21.googlecode.com/svn/trunk@3583 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Wrote doctests for MetadataBundle.write().


git-svn-id: http://music21.googlecode.com/svn/trunk@3584 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Updated a demo file to reflect changes in the metadata subsystem.


git-svn-id: http://music21.googlecode.com/svn/trunk@3585 5c16147e-a200-11de-b0cc-ab0ad15e60d0