1. Created net.simon987.server.crypto package and moved RandomStringGenerator there.
2. Created CryptoProvider class, added a global instance to GameServer for use by game entities.
3. Created interface for cyphers, abstract class for shift substitution cyphers, classes for no cypher, Caesar cypher, Vigenere cypher and autokey cypher.
4. Created some Crypto exceptions.
5. Removed static encryption/decryption methods from VaultDoor, and moved getRandomPassword() to the global CryptoProvider instance.
Added some code to VaultDoor, including random password generation and code to encrypt/decrypt under a Vernam-like cypher.
Modified door opening policy to include the possibility of keeping the door open.