Add support for yr.no as a weather provider

I have implemented yr.no (by the Norwegian meteorological institute) as a weather provider. You just need to add it to the preferences page, I think the name "yr.no" should be ok to use for it. 
I also changed the switch statement for the weather providers and moved the default: to the bottom since JSHint complained about it and I don't know if that really works or if it will catch all before it comes to the cases below.