Added more doctests for MetadataBundle set-wise operations.


git-svn-id: http://music21.googlecode.com/svn/trunk@3581 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Wrote more docstrings for MetadataBundle.


git-svn-id: http://music21.googlecode.com/svn/trunk@3582 5c16147e-a200-11de-b0cc-ab0ad15e60d0