So that work done on non-workday days could be seen in Workdays and Daily and Weekly reports...

* lib/Metiisto/Entry.pm
    - Modified find_week() to find the week from Sunday thru Saturday instead of
      Monday thru Friday
* lib/Metiisto/Util/DateTime.pm
    - Added sunday() class method
    - Factored out code from monday() and sunday() into a more generic method
      called day_of_the_week() that can find the date for any day of the week
      and changed monday() and sunday() to use it.
* lib/Metiisto/Workday.pm
    - Modified find_week() to find the week from Sunday thru Saturday instead of
      Monday thru Friday
* t/lib/Metiisto/Util/DateTimeTest.pm
    - Added tests for day_of_the_week() and sunday()