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 folder named `/viewerjs-x.y.z` which then contains all files directly.
I can tell you that confuses a lot (e.g. when doing live coding 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. Example: `/viewerjs-0.2.1/ViewerJS/`
Keeping the toplevel folder as it is matches usual packaging patterns and avoids overwriting similar directories when unzipping, due to the namespacing.

Additional advantage: in the future we should put README and Howtos below the toplevel folder as well, those would then be nicely separated from the actual code folder.