src/notes/notes.h: add get/set for release drop level (closes: #203)
src/notes/notes.h: add aubio_notes_{get,set}_minioi_ms
src/notes/notes.h: add doxygen header
src/notes/notes.h: add _{get,set}_silence methods
src/notes/notes.h: clean up prototypes for python generator
src/notes/notes.c: complete implementation, now equivalent to examples/aubionotes.c
src/notes/notes.c: add notes object basics