Added csrf token

Usage:

```php

<?php
$form = new Form();

// enable csrf passing key used for $_SESSION and $_POST
$form->enableCsrf('csrf_token');

// add csrf token into hidden field
$form->csrfToken();

$form->text('username');
$form->password('password');
$form->password('confirm_password');

$form->submit('submit_button', 'Click to Continue');

// when post is submit, check token
if($form->checkCsrf()) echo 'csrf passed';
else echo 'csrf failed';

// optional we can add timestamp, so token expire after X minutes
$timespan = 10; // 10 minutes
$multiple = true; // use token multiple time, usefull when ajax submission
$throwException = true; // trow an exception if failed
if($form->checkCsrf($timespan, $multiple, $throwException))
...

```

Inspired by https://github.com/BKcore/NoCSRF