Features
   * Added partial support for parsing the PKCS #7 Cryptographic Message
     Syntax, as defined in RFC 2315. Currently, support is limited to the
     following:
     - Only the signed-data content type, version 1 is supported.
     - Only DER encoding is supported.
     - Only a single digest algorithm per message is supported.
     - Certificates must be in X.509 format. A message must have either 0
       or 1 certificates.
     - There is no support for certificate revocation lists.
     - The authenticated and unauthenticated attribute fields of SignerInfo
       must be empty.
     Many thanks to Daniel Axtens, Nayna Jain, and Nick Child from IBM for
     contributing this feature, and to Demi-Marie Obenour for contributing
     various improvements, tests and bug fixes.