added remove method for Issues (#149)

Author: brammeleman

lib/Gitlab/Api/Issues.php

@@ -53,6 +53,16 @@ public function update($project_id, $issue_id, array $params)
         return $this->put($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_id)), $params);
     }
 
+    /**
+     * @param int $project_id
+     * @param int $issue_id
+     * @return mixed
+     */
+    public function remove($project_id, $issue_id)
+    {
+        return $this->delete($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_id)), $params);
+    }
+
     /**
      * @param int $project_id
      * @param int $issue_id