generated zip archives now support utf8 file names

refs #1086, #930, #578

File/folder names:
中文blah中文blah.txt
undaçao.doc
öäüß.txt

Currently known issues/observations:
- Chinese characters are not displayed within my Gnome Archive Tools
- On Windows XP with built in zip support the file and folder names are totally screwed

Next steps:
- either generate zips depending on the client os
- or normalize the file names to ASCII :anguished: 

@owncloud/core-developers 
I need reviewers and tester on Windows, Mac and Linux

@skodak thanks a lot for relicensing your code! 
Can I ask you to add a comment on this PR where you state again that you agree to relicense? THX