Add encrypt() and decrypt() methods.

These methods use AES-256 and provide a simple to use API with easy to
remember names.
Add support for aes encrypted cookies.

With Security supporting AES encryption it is also ideal to have AES
compatible cookies. Refactor and simplify code. Dynamic invocation of
static method is a bit obtuse and the various methods don't all have the
same arguments.
Fix compatibility with PHP 5.2
Add hmac to encrypted data.

Using an HMAC ensures that the ciphertext has not been
modified.
Fix coding standards error.
Merge pull request #1568 from markstory/2.5-encrypt

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