Merge in context.jsonld to registration schema

[jesserobertson]

Would be good to have the JSON-LD context that @fils has worked up sitting alongside the JSONSchema defns and some tests to ensure that these are in sync.

To do:

• [x] Copy over the existing work from @fils in the context folder into the schema.igsn.org tree
• [ ] Build a set of unit tests which use pyld to build the JSON-LD representation and check that everything is valid
• [x] Add the @id tags to the JSONSchema (@fils is there anything else we need in the straight schema checks to make sure that things are valid on the graph side?)
• [x] Finish updating examples/examples.md with the @id tags
• [ ] Get @fils and @dr-shorthair to review...

[jesserobertson]

@fils has already updated https://github.com/IGSN/igsn-json/blob/master/schema.igsn.org/json/registration/v0.1/context.jsonld for me :)