Attach page images to any engine

I wanted to be able to use page-images for my own custom engines. I took a look at how page-images was used for refinerycms-blog posts and decided it would be possible to make the code generic to attach to arbitrary engines.

I chose a configuration variable that could be set in the page_images initializer to determine to which engine the page-images would be attached, and in which admin tab the images could be added. I added Refinery::Page and Refinery::Blog::Post as defaults to match how refinerycms-page-images currently works. Haven't added generic tests, I'm not sure what approach to take there, as currently the blog post test is in a separate file.

Good idea, yes or no?