Update usage.rst

added guide for how to access dimensions. Took me ages to figure it out so I thought it would be nice if it appeared somewhere in the guides.
Removed verbose storage object from function save_thumbnail
Fixed: sending signal thumbnail_created after having created highres thumbnail
Bug fixed: files in high resolution (@2x) are not deleted
Added signal handler which optionally optimizes thumbnails using external programs
Fixed a typo
Made utf-8 safe
Zen of Python: EAFP
Log less if everything is OK
Test setting with optimizing
Added documentation for image optimization
Changed Posix mode to 755
Fixes upstream #37: add support for Indexed PNG files.
Better documentation for THUMBNAIL_DEFAULT_STORAGE
Using src in img tags
Update thumbnail.py
Merge pull request #289 from Siecje/master

Use src rather than href in img tag examples
Cache the thumbnail info before calling the signal
Fix thumbnail_exists to not use modtime for remote storages

Thanks to @katzlbt for pointing this out in #293
Remove a redundant logic check