Updates for dutch text

I also had a look over the entire site in dutch now and enhanced some translations. Doing this I found some english strings which are probably not yet in the translation engine. You probably just have to add the first list to the config files, but the second list will be more work. You are probably very well aware that most of these plugin things aren't yet translated ;-)

Easy ones!
* Online Payments (whole page)
* New Invoice : Recurring
* All notifications on the dashboard
* Clients Create : Add/Remove Contact , Cancel, Save
* Industries

Strings with possible technical implications, only datepicker seems really important to me.
* Datepicker
* Countries
* Pagination in all tables : Showing 1 to 10 of 20 
* Choose File / File size
* Invoice Ninja logo: Created by - allthough everyone understands this I guess :-)
* Template names
* Languages, could be avoided by specifying each language in its native language.
* Payment terms: I'm not familiar with Net30, we would just indicate a date like withing a month. But it  looks like this is exactly what Net30 is. So no real issue, but could be weird for some people.