Support data structure referencing instead of using dereferenced version

[kylef]

This is an incomplete pull request to support referenced data structures.

This has various benefits:

• Adds support for circular referenced schemas
• Decent performance boost when there are lots of referenced schemas (seeing ~10 seconds shaved on a ~27 second Swagger document)
• Adds data structure section
• DRYies up the resultant parse result significantly

Remaining Tasks

• [ ] Changelog
• [ ] https://github.com/apiaryio/fury-adapter-swagger/pull/130#discussion_r137386128
• [ ] Test ref in all cases
  • [ ] allOf needs additional support
  • [ ] oneOf

[pksunkara]

Looks good as a POC