Create Provider Model in unglue.it

[eshellman]

As we add more ebooks, we may want to generalize the way we handle Ebook.provider . See Gluejar#363 (comment)
Notes at https://www.evernote.com/shard/s1/sh/05a86843-12f0-45fe-a7dd-562b76d06365/79c73b575f3b5f7afc6b4bc85f5f5b6a