Add holidays for New Zealand

[tenjaa]

Hey,

I saw New Zealand was missing, so I added the statutory holidays as described in the law.
Wikipedia summarizes them also pretty nice.

While writing this, I just saw that DAY_AFTER_NEW_YEAR and WAITANGI are unique, so I will add some translations for them.

And I copied from gb as you can see.. Will fix it too

Please tell me if I missend anything!

References:
http://www.legislation.govt.nz/act/public/2003/0129/latest/DLM237120.html#DLM237120
https://en.wikipedia.org/wiki/Public_holidays_in_New_Zealand

[coveralls]

Coverage remained the same at 84.533% when pulling b78c3f6 on tenjaa:master into 3ee3df6 on svendiedrichsen:master.

[coveralls]

Coverage remained the same at 84.533% when pulling a51b853 on tenjaa:master into 3ee3df6 on svendiedrichsen:master.

[tenjaa]

Everything I saw should be fixed.

Waitangi Day is never translated if you look through Wikipedia or the rest of the internet. So I left it as it is in every language.

Would be cool to get some feedback :)

[svendiedrichsen]

@tenjaa As far as I can tell it looks very good. Thank you very much.

[tenjaa]

Cool! 👍

Do you have an idea when to release the next version and we can expect these changes on maven central?

[svendiedrichsen]

I had no plans but I will do it any day soon now.

[svendiedrichsen]

0.5.3 is released. Have fun.