Updated version with working Calibre's ebook-convert tool

-  fixed issue that the execution of Calibre's `ebook-convert` didn't…

… worked correct,

   therefore changed the internal Calibre path from `/opt/calibre/bin` to `/opt/calibre`;
   thanks to @bodybybuddha and @adocampo for testing and finding a solution for this issue;
   see for details at issue #28

New Alpine Version 3.8 and Calibre Tools (ebook-convert)

- new base image 3.8-1

- integrated Alpine glibc v2.28-r0
- integrated enhancements from jim3ma for Calibre ebook-convert
- updated README.md and CHANGELOG.md

Update Release

- 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

Configuration directory support

v1.1.10

- added support for usage of a configuration directory

Bugfix related to PGID & PUID 0 alias root

 - new base image technosoft2000/alpine-base:3.6-3

 - supports also root as user and group via PGID and PUID value 0 correct

Bugfix related to kindlegen

 - added additional check to proof if the `vendor` directory is avail…

…able and create it if needed

Support of container startup without existing Calibre library

 - initial files like 'metadata.db' will be now only added if write a…

…ccess exist

Usage of ImageMagick 6

 - fixed issue with ImageMagick and Wand

 - usage of self compiled ImageMagick 6 instead of Alpine provided ImageMagick 7

Fixed issue #9 and additional enhancements

 - corrected README.md and CHANGELOG.md because of wrongly used issue…

… number (typo)

 - ticket #7 is still in work and ticket #9 is fixed

Fixed issue #5 and #6

 - fixed issue #5 - Unable to create /tmp/Mobi

 - fixed issue #6 - Not possible for symlinks to be created