#31 merge global local schema

[KajKandler]

John, I had a look at the global/local schema merge issue #31, as I reported.

This change is rather convoluted:

• I had to write an odd document_loader for jsonld because their signature assumes that all schema is loaded from a URL (it is a beta after all)
• I had to refactor quite some code, because the injection of schema does not work, after the entire schema is converted to a "@graph", which is fine with me. Let me know if you'd rather not have this and I'll see if/how I can make this less restrictive (there are hints in the jsonld lib, but not sure how it works.