xCookie: Store and retrieve cookie values with a secret key

This class can store and retrieve cookie values with a secret key.

It can store values in given cookies using a secret key to encrypt the stored values.

The class can also decrypt a previously stored cookie value using the same secret encryption key.

The class can also check if a given cookie is set and delete cookie values.

The cookie lifetime, the encryption key and the cookie domain are parameters configurable using constant values.