Make it possible to get and set superseding schema info

[spenes]

We are adding a new feature to Enrich to make it possible supersede schemas. This feature uses the $supersededBy field in the Iglu schema. It should be possible to set this field while uploading new schema to Iglu Server.

Superseding info can be submit either via $supersededBy field or $supersedes field. Let's explain these with examples:

 "$supersededBy": "1-0-8"
  "self": {
          "vendor": "com.acme",
          "name": "test",
          "format": "jsonschema",
          "version": "1-0-7"
  }

In this example, 1-0-7 version is superseded by 1-0-8.

  "$supersedes":[ "1-0-4", "1-0-5", "1-0-6"]
  "self": {
          "vendor": "com.acme",
          "name": "test",
          "format": "jsonschema",
          "version": "1-0-7"
  }

In this example, 1-0-4, 1-0-5 and 1-0-6 are superseded by 1-0-7.

When a superseded schema schema fetched, irrespective of how that superseding info is submitted, it will contain $supersededBy field.