Add ability to get user by ID

Based on http://stackoverflow.com/questions/11976393/get-github-username-by-id

Author: bocharsky-bw

lib/Github/Api/User.php

@@ -43,6 +43,18 @@ public function all($id = null)
         return $this->get('users?since=' . rawurldecode($id));
     }
 
+    /**
+     * Get extended information about a user by its ID (UNDOCUMENTED).
+     *
+     * @param int $id the user ID to get
+     *
+     * @return array informations about the user
+     */
+    public function get($id)
+    {
+        return $this->get('user/'.rawurlencode($id));
+    }
+
     /**
      * Get extended information about a user by its username.
      *