Calendar class and methods created and removed

- Locale based calendars introduced rather than generic
  HolidayCalendar(). This will allow additional calendars to be added
with greater ease: create new locale and add new method for
are_good_days() etc.
- Defined `are_good_days()` for `SydneyCalendar`.
- Added `easter_monday()` function
- Removed unnecessary `roll()` method.