Extract serializer logic into lib

To use the same code in hubot I extracted the serializer logic into lib to later move it into `truffle-kredits` when we are done with all other common logic.

tests/unit/models/contributor-test.js

@@ -1,6 +1,4 @@
 import { moduleFor, test } from 'ember-qunit';
-import schemas from 'npm:kosmos-schemas';
-import tv4 from 'npm:tv4';
 
 moduleFor('model:contributor', 'Unit | Model | contributor');
 
@@ -10,17 +8,3 @@ test('#avatarURL() returns correct URL', function(assert) {
 
   assert.equal(model.get('avatarURL'), 'https://avatars2.githubusercontent.com/u/318?v=3&s=128');
 });
-
-test('#toJSON() returns a valid JSON-LD representation of the model', function(assert) {
-  let model = this.subject();
-
-  model.setProperties({
-    name: 'Satoshi Nakamoto',
-    kind: 'person',
-    github_uid: 123,
-    github_username: 'therealsatoshi',
-    wiki_username: 'Satoshi',
-  });
-
-  assert.ok(tv4.validate(model.toJSON(), schemas['contributor']));
-});