Add token factory.

Simplify token creation by using a factory, instead of doing:

```php
<?php

        $doneToken = $tokenStorage->createModel();
        $doneToken->setPaymentName($paymentName);
        $doneToken->setDetails($storage->getIdentificator($paymentDetails));
        $doneToken->setTargetUrl(
            $this->createAbsoluteUrl('paypal/done', array('payum_token' => $doneToken->getHash()))
        );
        $tokenStorage->updateModel($doneToken);

        $captureToken = $tokenStorage->createModel();
        $captureToken->setPaymentName('paypal');
        $captureToken->setDetails($storage->getIdentificator($paymentDetails));
        $captureToken->setTargetUrl(
            $this->createAbsoluteUrl('payment/capture', array('payum_token' => $captureToken->getHash()))
        );
        $captureToken->setAfterUrl($doneToken->getTargetUrl());
        $tokenStorage->updateModel($captureToken);

        $paymentDetails['RETURNURL'] = $captureToken->getTargetUrl();
        $paymentDetails['CANCELURL'] = $captureToken->getTargetUrl();
        $storage->updateModel($paymentDetails);

```

we just do:

```php
<?php

$payum->getTokenFactory()->createCaptureToken(
    $paymentName, 
    $paymentDetails, 
    'paypal/done'
);
```

@alexandernst Could you please test it since you have working app?