Update nl.properties

Original translation was already good, I changed some lines to make it a bit more fluent and corrected some small mistakes. Also added capitals to the beginning of each sentence and in case of fragments used capitals for each word (e.g. Next Page i.s.o. Next page) to increase consistency. 

Shouldn't we use the more formal 'u' in stead of 'je'? I understand the arguments for both, but when in doubt I prefer a more formal version. Also this is more normal in Flemish, who will also use Dutch localisation.