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