[Suggestion] Restructure source code to make integration easier

Hi team

Firstly, thanks for a good service. We are using cloudinary gem at Envato and it has been smooth.

When I look into the source code of this gem, I find that there was a strong Rails integration (which is fine), however I think this gem can/should be used by other non-Rails framework too (or even pure Ruby). So I am wondering what do you think about splitting the non-Rails code into a core component and having a separate `lib/cloudinary/rails.rb` to include the core + rails integration. I believe this would allow other frameworks to develop integration with cloudinary without having to manually remove the rails integration part.

I am happy to provide PR if your team think it aligns with your product roadmap.