More backward compatible image_info option processing

Some older (but useful) sorl engines may not pass 'image_info' to write method. This lead to KeyError when using with sorl >= 11.12.1b
Merge pull request #303 from GeyseR/patch-1

More backward compatible image_info option processing
fix #309 when THUMBNAIL_PRESERVE_FORMAT=True and remote images
Merge pull request #310 from mgaitan/fix309

fix #309, fix _get_format for images with urls
Merge branch 'master' of https://github.com/mariocesar/sorl-thumbnail