In Style2CSS do not hardcode the fallback for font-family to sans-serif

Fixes the sometimes difference in what substitution font is used in the
font selector tool and what substitution font is used in the ODF document
display for fonts which are not installed on the system
Merge pull request #27 from kossebau/doNotHardCodeSansSerifAsFontFamilyFallBack

In Style2CSS do not hardcode the fallback for font-family to sans-serif