Format Preservation with minimal alterations

This pull request adds the ability to preserve the file type of the image being converted into a thumbnail depending on the boolean settings.THUMBNAIL_PRESERVE_FORMAT, which is set to False by default.

This pull request gracefully degrades and defaults to using settings.THUMBNAIL_FORMAT. Applications that currently use sorl should not notice any difference in functionality unless settings.THUMBNAIL_PRESERVE_FORMAT is set explicitly set to True.

This pull request is desirable for anyone developing a medium scale application that relies on sorl, as the dilemma of preserving transparent PNGs as PNGs while optimizing JPGs arises quickly. I have several colleagues who have written this functionality into their application or have created their own backend to handle this case; I feel this is basic functionality that should be included in the sorl repo.

Thank You and Best Regards,
~pygeek