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
Better documentation for THUMBNAIL_DEFAULT_STORAGE
Merge pull request #265 from jrief/postprocessor

Postprocessor

Conflicts:
	easy_thumbnails/files.py
added textfixtures to test dependencies
added textfixtures to test dependencies
using .format() for string formatting
special test settings for postprocessor are part of the unit tests now
added special unit tests for postprocessor optimizer
Merge remote-tracking branch 'upstream/postprocessor' into postprocessor