Add NullEncryption and simplify Cipher creation

net.secloud.core.security.CryptographicAlgorithms now has a method
createCipher. net.secloud.core.security.SymmetricEncryptionParameters
key and iv fields type have been changed to Option[_] to allow
algorithm that do just use a key or neither a key or an IV (like
net.secloud.core.security.NullEncryption).