CPasswordHelper

The motivation and background for this component is exactly the same as described in PHP RFC [Adding simple password hashing API](https://wiki.php.net/rfc/password_hash), so I shall not explain that.

This class is different from what may be coming in PHP 5.5:

* it works with PHP 5.1+ rather than 5.5+
* it is written as a class rather than a set of functions in the global namespace
* it doesn't trigger_errors, it throws CException where necessary
* it is a Yii CApplicationComponent
* it has fewer options
* it has documentation