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)
Fix middleware url regexp
Update README
Version bump, getting ready for 1.0
Add changelog
Clarify Readme
Clean up Known Issues portion of Readme