Added cloning of encryptor/decryptor

[lgoldstein]

The expected use-case is as follows:

• User sets up a "template" encryptor/decryptor - cipher, compression, keys, etc.
• There are several threads that according to some input and/or business logic apply the template with some minor changes - e.g., different symmetric password, different compression, with/out armored headers, etc.
• Since an encryptor/decryptor is stateful, the threads cannot share the same instance so they need to create a new one to be used locally by each thread.
• By providing clone() method implementation we simplify the code since threads do not need to re-initialize the encryptor/decryptor from scratch (which could be time consuming) - only modify the specific settings they need locally
• Furthermore, by using the clone() method we ensure that users don't have to copy each setting individually from the template to the local copy - thus avoiding possible errors by forgetting to copy a field, failing to create a new copy for shared compound objects references (e.g., Ring, armored headers, etc.) or not keeping up with future changes by not copying new fields we might add.

[justinludwig]

Thanks -- looks good! I think you might also want to implement Cloneable for Key and Subkey, too, though. Otherwise, if you add keys to an encryptor/decryptor ring before you clone it, you'll end up with multiple encryptor/decryptors sharing the same subkey instances -- which could would cause problems if clearSecrets() is called on one encryptor/decryptor while other encryptors/decryptors are still using a shared subkey instance (as it will clear the subkey's extracted private key material, as well as any passphrase that could be use to extract it again).

[lgoldstein]

Good catch - done.