Moved holiday to the top and defined 'New year' as a Holiday.
Moved fixed definition construction to Holiday class. Now allow FIXED_HOLIDAYS to contain Holiday entries.
Update United States fixed holidays to use Holiday definitions.
Update UnitedStates variable days with Holiday instances

--HG--
extra : amend_source : f15d243f1146ebf29f8970f2e3159be9265df632
Fix flaws in Holiday.replace
Must copy name also
Correct two-tuple compatibility
Construct Holiday instances from variable holidays as well.
Add support for addition of a Holiday
South Africa now uses get_fixed_holidays, simplifying the implementation.