`AWS::S3::Base.establish_connection!` is not on the `aws/sdk`, and this code doesn't get used anymore.

This reverts commit 58671ebf.

I am reverting this because the tests broke and because the code itself
looks hideous.

Fixes #743

Fixes #745

In the effort of decoupling Rails from unit specs, you can't have any code that calls `Rails` (since it isn't available). In my unit specs that need Paperclip, I call `Paperclip::Railtie.insert` and all is good :)

Introducing `:restricted_characters` in Paperclip::Attachment.default_options  so people can override their blacklist characters by override that setting.

Fixes #703

Closes #678. Closes #450.

If a model has an existing attachment and styles, and later a new attachment is added (not just new styles), "rake paperclip:refresh:missing_styles" would crash with "TypeError: can't convert nil into Array".

#fingerprint now consistently returns nil before and after saving an attachment if the value can't be written to the db.