Add challengePassword encryption/decryption for non RSA recipients.

[netcat-pl]

Adds challengePassword encryption/decryption for non RSA recipients. This solves issue #300.

The goal was to minimally modify the existing interface, so all public API prototypes were retained. Only the DefaultCallbackHandler class has been extended to allow for providing the required challenge password values. Here is an example:

CertificateVerifier verifier = new ConsoleCertificateVerifier();
Map<String, String> passwords = new HashMap<>();
passwords.put("CA1", "secret1");
passwords.put("CA2", "secret2");
CallbackHandler handler = new DefaultCallbackHandler(verifier, passwords);

More details here.

Does this pull request add a feature or solve a bug? Yes, it does. It's a new feature PR.

[warrenmnocos]

Thanks! Waiting for this to get merged.