Fix #87: security helper converted into component and improved

Fix #87: security helper improvement

Migrated from #4052
To make more clear that the following is not recommended but only for apps that must reuse cyphertexts generated with the previous version:

Yii has upgraded its default encryption and hash parameters since Yii 2.0 Beta. If you need to decrypt/validate data that was encrypted/hashed before this upgrade, use the following configuration of the 'security' component: