Refactor/simplify methods

[lauft]

• Use explicit class-based calls for static methods
• Use date methods
• Extract 'date' function, encapsulate usage of SmartDayArrow in utils.py
  • Introduce class SmartDayArrowWrapper to provide test functions for dates
  • SmartDayArrow shifters now only accept a callable returning a date
  • HolidayGenerator.on(...) now only accepts a callable returning a date
  • Replace static date-shifting methods with condition checks from utils.py.
• Simplify date methods
• The 'Monday on or first Monday following the 1st of ' is equivalent to
  the 'first Monday of '.