Add support for OCB mode as per RFC7253

Reviewed-by: Tim Hudson <tjh@openssl.org>
Add EVP support for OCB mode

Reviewed-by: Tim Hudson <tjh@openssl.org>
Add tests for OCB mode

Reviewed-by: Tim Hudson <tjh@openssl.org>
Add documentation for OCB mode

Reviewed-by: Tim Hudson <tjh@openssl.org>
Added OPENSSL_NO_OCB guards

Reviewed-by: Tim Hudson <tjh@openssl.org>
Add CHANGES entry for OCB

Reviewed-by: Tim Hudson <tjh@openssl.org>