[Omnipay] allow user to add custom gateway.

Hi,
the current implementation doesn't allow to add unofficial gateway, I think we should find a way to add custom gateway.
The simplest way to solve this is to allow setting full class in param 'type' , here is an example:

```yaml
payum:
    contexts:
        paypal:
            omnipay:
                type: \Omnipay\PaypalRest\Gateway
                ...
```