Support for encrypted payload

[delitestudio]

User can set an (externally) encrypted payload as described in #22 to be used with Chrome.

[delitestudio]

Here it is an example on how to use it:

$sender = new PHP_GCM\Sender($api_key);
$payload = Minishlink\WebPush\Encryption::padPayload(json_encode($payload_data), false);

$encrypted = Minishlink\WebPush\Encryption::encrypt(
    $payload,
    $localPublicKey,
    $salt,
    version_compare(phpversion(), '7.1', '>=')
);

$message = new PHP_GCM\Message('push');
$message->publicKey($encrypted['localPublicKey']);
$message->salt($encrypted['salt']);
$message->rawData(base64_encode($encrypted['cipherText']));

$sender->sendNoRetryMulti($message, array($token));