2.5 - Add Security::encrypt() and Security::decrypt()

Add new methods to simplify access to AES-256. After talking with @lorenzo we agreed that `rijndael` is hard to remember, and has a funky signature taking the mode as a string. These new methods have simple names and use a more broadly accepted cipher set (AES-256).

You might notice that the algorithm is `MCRYPT_RIJNDAEL_128`, after reading both the PHP docs and [zf2 source code](https://github.com/zendframework/zf2/blob/master/library/Zend/Crypt/Symmetric/Mcrypt.php#L74) I'm fairly sure this is correct constant to use and that the key length changes the strength of the encryption.