DSSPrivateKeyEntry expose the wrapped private key

[vgalloy]

Motivations: Since DSSPrivateKeyEntry doesn't expose the wrapped private key, methods like SignatureTokenConnection#sign(ToBeSigned, DigestAlgorithm, DSSPrivateKeyEntry) need to cast the DSSPrivateKeyEntry into KSPrivateKeyEntry. This cast doesn't allow user to implements there own DSSPrivateKeyEntry.

Modifications:

• Add DSSPrivateKeyEntry#getPrivateKey
• Typo in DSSPrivateKeyEntry doc