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. 
@pjedrzejewski also with getFirstModel method we should not do this as long as we use that method.