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.

app/lib/kredits/index.js

@@ -0,0 +1,5 @@
+import ContributorSerializer from './serializers/contributor';
+
+export {
+  ContributorSerializer
+};

app/lib/kredits/serializers/contributor.js

@@ -0,0 +1,93 @@
+/**
+ * Handle serialization for JSON-LD object of the contributor, according to
+ * https://github.com/67P/kosmos-schemas/blob/master/schemas/contributor.json
+ *
+ * @class
+ * @public
+ */
+export default class Contributor {
+ /**
+  * Deserialize JSON to object
+  *
+  * @method
+  * @public
+  */
+  static deserialize(serialized) {
+    let {
+      name,
+      kind,
+      url,
+      accounts,
+    } = JSON.parse(serialized);
+
+    let github_username, github_uid, wiki_username;
+    let github = accounts.find((a) => a.site === 'github.com');
+    let wiki   = accounts.find((a) => a.site === 'wiki.kosmos.org');
+
+    if (github) {
+      ({ username: github_username, uid: github_uid} = github);
+    }
+    if (wiki) {
+      ({ username: wiki_username } = wiki);
+    }
+
+    return {
+      name,
+      kind,
+      url,
+      github_uid,
+      github_username,
+      wiki_username,
+      ipfsData: serialized,
+    };
+  }
+
+ /**
+  * Serialize object to JSON
+  *
+  * @method
+  * @public
+  */
+  static serialize(deserialized) {
+    let {
+      name,
+      kind,
+      url,
+      github_uid,
+      github_username,
+      wiki_username,
+    } = deserialized;
+
+    let data = {
+      "@context": "https://schema.kosmos.org",
+      "@type": "Contributor",
+      kind,
+      name,
+      "accounts": []
+    };
+
+    if (url) {
+      data["url"] = url;
+    }
+
+    if (github_uid) {
+      data.accounts.push({
+        "site": "github.com",
+        "uid": github_uid,
+        "username": github_username,
+        "url": `https://github.com/${github_username}`
+      });
+    }
+
+    if (wiki_username) {
+      data.accounts.push({
+        "site": "wiki.kosmos.org",
+        "username": wiki_username,
+        "url": `https://wiki.kosmos.org/User:${wiki_username}`
+      });
+    }
+
+    // Write it pretty to ipfs
+    return JSON.stringify(data, null, 2);
+  }
+}