- exposed additional directories (see ticket #24)
- added support for setting the Calibre Web application folder (see ticket #27)
- updated python `requirements` and `optional requirements`at dockerfile
- added Container Directory Structure information at README.md (see ticket #24)
- kindlegen archive will be downloaded on container startup into `/calibre-web/kindlegen` and is not directly included in the image anymore
- ownership of symlink of `app.db`, `gdrive.db` and `kindlegen` is changed from _root_ to the _calibre_ user & group
