Put all files into a subfolder ViewerJS/ in the zip

All examples on viewerjs.org talk about using a folder named "ViewerJS". But the zip currently is structured to have a toplevel dir named "/viewerjs-x.y.z" which then contains all files directly.
I can tell you that confuses a lot (e.g. when doing live demos during talks ;) ).

To spare people renaming that toplevel folder to ViewerJS to match the examples, instead that folder should exist already below the toplevel folder and contain all files as expected. So people can just `mv` that folder around, avoiding any typos.
Additional advantage: in the future we should put README and Howtos below the toplevel subfolder as well, those would then be nicely separated from the actual code folder.