v0.2.0

• Added a chrono feature for enabling conversions to & from chrono types
• Gave MonthShape a calendar() method for obtaining the associated Calendar
• Gave MonthShape nth_date() and dates() methods for obtaining Date objects belonging to the month
• New Date methods:
  • pred() — returns the previous date
  • succ() — returns the next date
  • later() — returns an iterator over all later dates
  • earlier() — returns an iterator over all earlier dates
  • and_later() — returns an iterator that yields the receiver and all later dates
  • and_earlier() — returns an iterator that yields the receiver and all earlier dates
• Added a Weekday enum and a Date::weekday() method