Changed cookie encryption to rijndael

Since suhosin disabled srand() by default and CakePHP's Security::cipher()
function uses srand(), the encryption of cookies does not work properly out of
the box.

The cookie encryption is changed to rijndael and mcrypt extension must be
installed.

See also

- http://stackoverflow.com/questions/6087463/cakephp-securitycipher-is-not-working-specifically-on-server
- http://milesj.me/blog/read/security-cipher-suhosin