Fix #87: security helper improvement

Fix #87: security helper improvement

Basic fix to improve cryptographic strength `yii\helpers\Security`. Basic recomendations are applied.
Method `generatePasswordHash()` still requires improvement to be compatbale with `password_hash()` PHP 5.5 function.

Helper structure reworked to support BC (see UPGRADE.md)

Note: from the way I see this class is better to be an application component, then a static helper. I can make such coversion if the rest of the team agree with it.