MusPy v0.1.0

• Fix name of Chord.pitches_str (#3, 536a877) and types of Metadata.creators and Music.downbeats (ed2ed1f)
• Fix method _validate() of TimeSignature, Note and Chord (3504be3, ed2ed1f)
• Add wrapper functions list_datasets() and get_dataset() for datasets (9aacae0)
• Improve Base (121e718) and Dataset (2f24836)
• Make velocity attributes optional in Note and Chord (3504be3)
• Make ignore_exception in Dataset.convert() default to True (3504be3)
• Update dataset information (67fc563) and their md5/sha256 checksums (5821827)