change pdf command to prince
update styles for prince
add version to celery package as required for pyramid_celery
also replace space in filename with _ character
Merge branch 'prince' of https://github.com/voer-platform/vp.transformer into prince
convert material's version to string before join
update TOC, section titles and cleanup
update title page's styles, counter and section titles styles
Merge pull request #2 from voer-platform/prince

merge Prince to Master