v1: entity class naming

[arkid15r]

Currently there is no standard on entity holiday class naming. In general it somehow aligns with a country name. The idea is to use unified approach on entity class naming. For iso3166 entities it could be based either on alpha-2 or alpha-3 code, e.g. UsHolidays or UsaHolidays. The classes should still be available via their alpha-2 (and probably alpha-3) code aliases so for example US() and USA() imports would also work.

[arkid15r]

See #998 (comment) for the original discussion.