Update link in documentation for :s3_permissions.

Use hexdigest to ensure that tempfile names will be a reasonable length.  Remove illegal filename characters check since it is no longer relevant when doing a hexdigest on the filename

When using S3 for storage, their data consistency model makes the
current behaviour of Attachment#reprocess! unreliable.

Apparently, Amazon can not guarantee that PUT's called after DELETE's
is actually executed in that order.

Hence, I propose preserving files, when doing a reprocces.

This ensures that multiline strings are properly matched in their
entirety.

document :s3_credentials being able to take a Proc

Remove references to closed mailing list

http://robots.thoughtbot.com/moving-open-source-project-mailing-lists-to-stack-overflow/

Added changes to instructions to work with Rails 4's strong parameters.

Added 3.2.15 Rails to run test.

removes duplication