Case build-in classes

Ref: http://php.net/manual/en/reserved.classes.php
In php allowed use any case of build-in class:

$exception = new Exception('Message');
$exception = new EXCEPTION('Message');
$exception = new eXcEpTiOn('Message');

Always return exception build-in class.
Should we prettify class names? Also don't forget about namespaces.

1. File without namespace - new Exception() === new \Exception();.
2. File with namespace - new Exception !== new \Exception.

In theory we can do it:

1. If namespace exists only correct new \Exception();
2. If namespace doesn't exists correct and new Exception() and new \Exception();

首先检查 issue 中的 PHP 示例，以及其中引用的保留类和命名空间规则。完成标准是：就如何处理有命名空间和无命名空间文件中的内置类名，达成一项考虑命名空间的共识决定，随后在相关 formatter 测试中完成实现并补充覆盖。