Enforce agnosticism towards ActiveRecord; use ActiveModel

Paperclip is already agnostic towards ActiveRecord, i.e. it checks that AR is defined before including AR-specific code.

This commit does two things to enforce this agnosticism:
- Remove AR as a production gem dependency, make it a dev (test) dependency instead.
- Change comments to reflect AR vs. AM where appropriate

Lastly, although I have **not** included it in this PR, I would urge that Mongoid support (i.e. the `mongoid-paperclip` gem) be merged into the core of Paperclip. It is straightforward to do since Mongoid uses ActiveModel. I'd be glad to do this work if its acceptable to thoughtbot team.