added translation for the activerecord error messages that occur when the user tries to save a new invalid job offer / chair / student. In the course of this commit the template on how to create yml files for models changed! The README file for locales is updated and also the wiki entry on internationalization. Instead of t("job_offer.title) now one has to call t("activerecord.attributes.job_offer.title"). Through this change it is possible to use already translated yml files from public repositories in order to translate error messages containing the attribute names of models.