Use Rack middleware to generate missing dynamic attachment styles

- Remove Rails Engine
- Remove AttachmentStylesController
- No longer need to define routes when attachments are defined
- Remove Rails dependency entirely
- Insert new middleware, DynamicPaperclip::AttachmentStyleGenerator, when Rails is present
- Register all dynamic attachments in dedicated DynamicPaperclip registry
- Scan all incoming requests for paths matching registered attachment URL's
  and generate missing styles when needed (that logic is largely unchanged
  from what was formerly in the AttachmentStylesController)