Possible solution for issue #175

This is a proof of concept how this issue could be solved.
If ``settings.THUMBNAIL_HIGH_RESOLUTION`` is ``True`` (default), always two images are generated, the default one and a high resolution thumbnail.
The HTML code always references the default one. This avoids caching issues, since HTML pages very often are cached.

Now, if the client uses this Javascript library http://retinajs.com/ all img src tags are rewritten to the high resolution image if the client runs on a high quality display. Otherwise, or, if the hq-image can't be found, the default images is loaded.

This solution also avoids all caching issues for images.