0.2.0a5

added

• Sequence instances have attributes: labels, onsets_s, offsets_s, onsets_Hz,
  offsets_Hz, and file.
• Explanation of default to_csv function for user formats in howto-user-config.

changed

• Sequence class totally re-written
  • no longer attrs-based
  • because of somewhat complicated logic for validating arguments that
    was necessary in init (to prevent user from creating a 'bad'
    instance.)
• Sequences are immutable. Idea is they are just connectors between
  annotation and whatever user needs to do with it so you shouldn't
  need to change any attribute values after loading annotation
• Segment also immutable (by setting frozen=True in call to attr.s decorator)
• Transcriber.init uses config.json instead of config.ini to read defaults
  • this makes init logic more readable since we don't have to convert
    user_config dict to strings and then back again; default config just loads as
    a dict from the .json file and we add the user_config dicts to it