Bundle configuration based on omniplay classes names

Modifies bundle configuration:

1. Configuration root is sylius_omnipay, replacing old sylius_payments

2. Available Gateways are retrieved using Omipay GatewayFactory::find

3. In configuration a valid gateway type must be provided, it has to match Omnipay name. Example
```
sylius_omnipay:
      gateways:
        paypal:
          type: PayPal_Express
          label: Paypal Express
        sagepay:
          type: SagePay_Direct
          label: Sagepay Direct
```

I am starting to work with this bundle, hope this modification won't break anything!