Release v2.3.0

Changes from v2.2.2 :

- don't generate subfolders (see #130).
- add comment support (see #134).
- on `ZipObject#options`, the attributes `date` and `dir` have been deprecated and are now on `ZipObject`.
- on `ZipObject#options`, the attributes `base64` and `binary` have been deprecated.
- deprecate internal functions exposed in the public API (see #123).
- improve UTF-8 support (see #142).