Add reformation day for Lower Saxony

[DysphoricUnicorn]

Beginning this year (tomorrow actually) Reformation Day is a holiday in Lower Saxony.
I would make a pull request (similar to the one for Schleswig-Holstein #106 ) but I can't promise I will have enough free time in the near future to do so.
It would be great if this could be implemented :)

Source (German)
https://www.zeit.de/gesellschaft/zeitgeschehen/2018-06/reformationstag-niedersachsen-neuer-feiertag

[stelgenhof]

Thanks for letting me know. I can implement this for you, but not sure if I can make it before tomorrow :)

Cheers! Sacha

[stelgenhof]

@Taxcamp Sorry a day late, but the changes have been implemented (in the develop branch). If there is any issue, just submit a new issue.

Cheers! Sacha