Payum 013 upgrade

Chanages:

* Payum: (changelog)[https://github.com/Payum/Payum/blob/master/CHANGELOG.md], [upgrade](https://github.com/Payum/Payum/blob/master/UPGRADE.md)
* PayumBundle: (changelog)[https://github.com/Payum/PayumBundle/blob/master/CHANGELOG.md], (upgrade)[https://github.com/Payum/PayumBundle/blob/master/UPGRADE.md].

The most import change is added `getFirstModel` method. I added it to `Generic` request. Check that you use that method instead of getModel one before upgrading your apps.