Alternative resolutions

Optionally create a n-sized thumbnail for every thumbnail sorl-thumbnail creates.
This very helpful for "retina"-like HDPI screens.

Credits go to: @relekang and @philipn (#126 #119)

Improvements since ##126:

* rebased on latest master
* fixed imagemagick support (tmp source now is kept around until all thumbnails are ready)
* ``THUMBNAIL_ALTERNATIVE_RESOLUTIONS`` is empty by default (performance-wise)
* ``default_options`` does not change, avoiding "thundering herd problem":https://github.com/mariocesar/sorl-thumbnail/pull/126/files#r9129227

This is somewhat close to what I'm using in production already (but not exactly same code), but I guess we can improve some points later in next patches:

* remove the need for tmp image for imagemagick (?)
* get rid of side-effects in ``_create_thumbnail`` (source gets changed, this might cause problems for alternative thumbnails)

Thnx to @relekang and @mariocesar for feedback.