Supports full certification path building

- Added GetSigningCertificate() and GetAllCertificates() methods to
Pkcs11RsaSignature class
- Marked GetCertificateAs*() methods of Pkcs11RsaSignature class as
obsolete
- Added CertUtils utility class
- Example code now builds full certification path for the signing
certificate if all required certificates are stored on the PKCS#11
device