Images on public sharing get downscaled to increase use experience - thi...

...s will speed up loading time

@jancborchardt @PVince81 @georgehrke 

TODO:
- [x] return jpeg thumbnails for jpegs
- [x] introduce caching of images with preserved aspect
- [x] Looks like this needs to be added in preview.php as well, then ?
- [x] Would be good to have this in the slideshow view as well, 

see https://github.com/owncloud/gallery/pull/30