Translation files added & Dutch translation

I've added translations files for the plugin so it can be translated
using poedit or some other PO file editor. Ideally this would be
combined with Transifex, but for now this will have to do.
Fixed localization issues

Made sure all localization calls use the same localization domain and
altered the load_textdomain so it grabs the right files. Tested it with
the Dutch translation and this works flawlessly.