Add support for retrieving a single notification info using his ID (#489)

* Add support for retrieving a single notification info using his ID

* Syntax

* StyleCI

* Apply fixes from StyleCI

* Fix input type

* Fix as said

* Apply fixes from StyleCI

* Remove undocumented

* Add documentation

Author: m1guelpf

doc/notification.md

@@ -36,3 +36,10 @@ $client->api('notification')->markRead(new DateTime('2015/01/01'));
 ```
 
 Marks all notifications as read up until the current date, unless a date is given
+
+## Get a single notification using his ID
+
+```php
+$client->api('notification')->id($id);
+```
+Retrieves single notification data using his ID.

lib/Github/Api/Notification.php

@@ -57,4 +57,15 @@ public function markRead(DateTime $since = null)
 
         $this->put('/notifications', $parameters);
     }
+    /**
+     * Gets a single notification using his ID
+     *
+     * @link https://developer.github.com/v3/activity/notifications/#view-a-single-thread
+     *
+     * @param int $id
+     */
+    public function id($id)
+    {
+        return $this->get('/notifications/threads/'.$id);
+    }
 }