Write more unit tests

[wojtekmaj]

• Calendar
  • displays a view with a given value when value is given
  • displays a view with activeStartDate when no value is given and activeStartDate is given
  • displays a view with today's date when no value and no activeStartDate is given
• MonthView, YearView, DecadeView, CenturyView
  • renders proper view when given activeStartDate
  • applies tileClassName to its tiles when given a string
  • applies tileClassName to its tiles conditionally when given a function that returns a string
  • renders tileContent in its tiles when given a node
  • renders tileContent in its tiles conditionally when given a function that returns a node
• WeekNumbers
  • renders static divs as children when not given onClickWeekNumber
  • renders buttons as children when given onClickWeekNumber
  • calls onClickWeekNumber function with proper arguments when clicked a week number (ISO 8601)
  • calls onClickWeekNumber function with proper arguments when clicked a week number (US)