e of font sans-serif instead of non-standard "sans"

CSS defined "sans" as font-family which is not a default font family in CSS.
This commit changes it to sans-serif which is the official name for the generic font family http://www.w3.org/TR/CSS21/fonts.html#generic-font-families
Merge pull request #92 from pauginer/font-family-fix

Use of font sans-serif instead of non-standard "sans"