music21 library, MacOS Automator service

A trimmed copy of the latest music21 was added. An Automator service called 'Convert to MusicXML' adds a contextual menu item for text files which generates a MusicXML file (via rtf2xml.py).
Conveniently renamed
Absolute imports, remove music21 embedding

The music21 embedding was real nonsense.
Update .gitignore
Clean-up of repo structure