#58 Disable AES 256 if not supperted

[Mirko]

By default current versions of the JDK have a deliberate key size restriction built in.
If you try to perform, say, 256-bit AES encryption with the default JDK, you'll find that it dutifully throws an InvalidKeyException, complaining with the not-too-explicit message "Illegal key size or default parameters".

Check if 256-bit AES is available, otherwise disable it.