Add client.cacheUser method (#548)

* Add client.cacheUser method

* Fixed ESLint issues

* Added existence check first

src/client/actions/UserGet.js

@@ -0,0 +1,15 @@
+const Action = require('./Action');
+
+class UserGetAction extends Action {
+
+  handle(data) {
+    const client = this.client;
+    const user = client.dataManager.newUser(data);
+
+    return {
+      user,
+    };
+  }
+}
+
+module.exports = UserGetAction;