WIP: Add helper class methods to get locale day names from stdlib

This approach for #55 (closed) adds utilities to the CalendarWeek utility class. It contains some magic to work with Django's locale. Also, it gets lazily evaluated in the model so it catches the request locale every time.

I could not test yet due to dayjob obligations. @hansegucker If you get around to it, please look at the idea and also checkout the branch in your environment to see if it breaks something.