Add Holiday class, which represents a holiday on a given date
Add replace method to allow year to be replaced
Load FIXED_HOLIDAYS as Holiday objects

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extra : amend_source : e2b1f36319875798bf21deb3075deab65f9ea230
Fix references to 'date' class and use super for invoking the constructor.
Added two-tuple compatibility to Holiday object

--HG--
extra : rebase_source : d0ec2a68e9565aaa015984de775b07df1d263189
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

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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.
Correct flake warnings that I don't get
One more instance of flake8
Use += for clarity
Update doctest to match implementation
Add test capturing the primary use case for the Holiday class - observance dates reflected in each result.