[WIP] LUBIS: Viewers based on tiled images

This changes the viewers from iipimage backend to tile based backend. It's fully functional and can be tested right away (if there are no tiles yet, the previews will not be available). However there are a couple of open issues that need to be solved before merge:

- We have to use the debug mode for the ga/ol3 API because of missing export in ol3. This is ready in ol3 but we need to update ol3 and our API with it.
- In the viewer, the current implementation creates requests for tiles that don't exist. This is ugly (I think it's because our images are not squared?). Maybe @oterral can have a look at this.
- To check for existance, I'd like to use the 'HEAD' command instead of 'GET' on the quickview/zero tile image. But the 'HEAD' command does not respond correctly on a reliable basis.

Thanks to @oterral for the great preparation in https://github.com/geoadmin/ol3/pull/75