Add X509Generator::sign()

        let mut p_key = PKey::new();

        p_key.gen(self.bits as usize);

        let x509 = try!(self.sign(&p_key));

        Ok((x509, p_key))

    }

    /// Signs certificate with a private key and returns it

    /// Note: That the bit-length of the private key is used (set_bitlength is ignored)

    pub fn sign<'a>(&self, p_key: &PKey) -> Result<X509<'a>, SslError> {

        ffi::init();

        unsafe {

            let x509 = ffi::X509_new();

            try_ssl_null!(x509);

            let hash_fn = self.hash_type.evp_md();

            try_ssl!(ffi::X509_sign(x509.handle, p_key.get_handle(), hash_fn));

            Ok((x509, p_key))

            Ok(x509)

        }

    }

}