Added option "-s"/"--sign": KEY.SF and KEY.RSA files are added to an existing JAR.

- "-s" must be followed by three mandatory arguments:
    * certificate file
    * private key file
    * JAR key alias (we use 'KEY' in the text below).
- Signature file KEY.SF is created from the MANIFEST.MF.
- Signature block file KEY.RSA is created from KEY.SF with external call to OpenSSL.