Alternative resolutions

Based on the work from #119, this pull request implements alternative resolutions for thumbnails.  Unlike #119, we allow custom resolutions and don't refer to them simply as "retina" (2x).

New setting value:

``THUMBNAIL_ALTERNATIVE_RESOLUTIONS`` (default ``[1.5, 2]``)

and a new template tag, ``resolution``, used as explained in the docs and in my comment on #119.