Add support for notes (#860)

* Add support for notes

* Ensure consistency with notes from ready payload

* Add getter method for users

* Minor tweaks

* Update warning messages

* More minor fixes

src/client/actions/UserNoteUpdate.js

@@ -0,0 +1,30 @@
+const Action = require('./Action');
+const Constants = require('../../util/Constants');
+
+class UserNoteUpdateAction extends Action {
+  handle(data) {
+    const client = this.client;
+
+    const oldNote = client.user.notes.get(data.id);
+    const note = data.note.length ? data.note : null;
+
+    client.user.notes.set(data.id, note);
+
+    client.emit(Constants.Events.USER_NOTE_UPDATE, data.id, oldNote, note);
+
+    return {
+      old: oldNote,
+      updated: note,
+    };
+  }
+}
+
+/**
+ * Emitted whenever a note is updated.
+ * @event Client#userNoteUpdate
+ * @param {User} user The user the note belongs to
+ * @param {string} oldNote The note content before the update
+ * @param {string} newNote The note content after the update
+ */
+
+module.exports = UserNoteUpdateAction;