Fixed #13605 -- Improved documentation of the django.core.files.storage...

Fixed #13605 -- Improved documentation of the django.core.files.storage module. Added documentation for DefaultStorage, get_storage_class, FileSystemStorage, and some missing public methods on Storage. New metadata targets included for everything. Thanks to kopernikus for the report and elbarto for contributing to the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14831 bcc190cf-cafb-0310-a4f2-bffc1f526a37