Made all corrections requested by @simon987 in the review to #119 . In addition, I've added some cyphers for use in vault doors.
Created net.simon987.server.crypto package and moved RandomStringGenerator there.
Created CryptoProvider class, added a global instance to GameServer for use by game entities.
Created interface for cyphers, abstract class for shift substitution cyphers, classes for no cypher, Caesar cypher, Vigenere cypher and autokey cypher.
Created some Crypto exceptions.
Removed static encryption/decryption methods from VaultDoor, and moved getRandomPassword() to the global CryptoProvider instance.
Made all corrections requested by @simon987 in the review to #119 . In addition, I've added some cyphers for use in vault doors.