Implemented a MetadataEntry class to help facilitate cascading searches and
parseable search results on MetadataBundle.

Searches on MetadataBundle now return new MetadataBundle instances, which can
themselves be searched.


git-svn-id: http://music21.googlecode.com/svn/trunk@3565 5c16147e-a200-11de-b0cc-ab0ad15e60d0