Added method User::refresh() to clear identity cache. (#46)

[Sikipiki]

• new feature Same identity from different namespaces #46
• BC break? no

Example in BasePresenter.php:

public function startup()
{
	$this->getUser()->getStorage()->setNamespace('namespace_1');
	.
	.
	.
	$this->getUser()->refresh()->getStorage()->setNamespace('namespace_2');
}

[Sikipiki]

After use, I found a problem with refresh(): each time the page loads, the session_id changes. This is caused by calling $this->storage->getIdentity() in method getIdentity() in Nette\Security\User.php.