completed / updated German language file (i18n\locales\de\messages.json)

Now 99% of the phrases are translated into German. I left open only two that are connected with migrating:

`"migrate_dlg_title" : { "message" : "Migrating Books" },`

and

`"migrating" : { "message" : "Migrating" }`

since I´m not sure about the context. What exactly does (or will do) this function? Export of the Readium library, import or both?

I also updated messages that have changed in the original US English version (for instance the copyright).