We need better circular reference identification

[whitlockjc]

Right now, circular reference identification doesn't seem to identify the following:

definitions:
  User:
    type: object
    properties:
      status:
        $ref: '#/definitions/Status'
  Status:
    type: object
    properties:
      user:
        $ref: '#/definitions/User'

This was originally brought up in #81.

[paulprogrammer]

Also, the circular reference feature does not work if you specify yaml output (-y) -- the same output that is rejected during JSON output is attempted (unsuccessfully) to be output in yaml.