:class interpolates to the plural

`s3_protocol` now returns the protocol without a colon. If you need a
colon, you can pass in `true` as a second argument.

This might not be a best solution, so any better patch that does not
introduce a regression is welcome.

Fixes #921

* Add StyleTokenNotFound in Errors
* Adjust tests to make sure :style present where not currently defined

Now it includes :class and :id_partition

Also, 3.1.0 and 3.1.1 will be yanked because of the regression.

@mvz caught this in 90ea9dab.
Thank you!

This will solve an issue when using Paperclip without Active Record
such as Mongoid.

Fixes #919

This is for Rails 3.1+

Fixes #902

Fixes #883

Re-fix of #838, #766, #911