Added support for mainnet transactions

[jasny]

$seedText = "...";
$account = (new LTO\AccountFactory('T'))->seed($seedText);

$node = new LTO\PublicNode('https://nodes.lto.network');

$amount = 1000e8; // Amount of LTO to transfer * 10^8
$recipient = "3Jo1JCrBvnWCg37VDxMXAjYhsS9rRDLBSze";

$transferTx = (new LTO\Transaction\Transfer($amount, $recipient))
    ->signWith($account)
    ->broadcastTo($node);