Normalize $ref / id behavior

[JanTvrdik]

The draft 5 should (finally!) normalize the behavior of $ref and id, both in text and in form of language-agnostic test files.

Some of the edge-cases were discussed e.g. in

• Proposal for next draft: Remove 'id' keyword json-schema/json-schema#77
• Rewrite dereferencing definition and URI vocabulary json-schema/json-schema#83
• https://github.com/json-schema/json-schema/wiki/$ref-traps
• https://github.com/json-schema/json-schema/wiki/The-id-keyword
• https://github.com/json-schema/json-schema/wiki/The-%22id%22-conundrum

[awwright]

A lot of the problems with "id" are a result of very poor definition. #50 should fix a lot of this, please review!

[awwright]

Because of recent commits and new related issues I'll close this out, if no existing issue exists feel free to comment or create a new issue. Thanks!