More options for Paperclip field, better development environment.

• Introduces the options thumbnail_style and big_style to control the image sizes that are displayed, by @klaseskilson.
• Better specs, better documentation, Simplecov, Rubocop and CI integration also by @klaseskilson.
• Adds a url_only option for attachments that are not images by @pedantic-git.

Check the updated README for instructions on how to use the new options.