image files can be generated differently on different systems
page html files contain the size of the image which can also be different on different systems

- added tests
- fixed issues found by shellcheck
- check for image files before generating directoy structure, so if none are
  found, nothing will be written
- do not print version number and dates in html files so that an execution on
  the same data always produces the same output which makes it easier to test
- removed version from script and --version option