Implemented get/setPaymentMethod and get/setIssuer from common

Replaces https://github.com/omnipay/mollie/pull/3

- [x] remove `setMethod()` from this gateway and replace with `setPaymentMethod()`
- [x] remove `setIssuer()` from this gateway and use default `setIssuer()` from common package
- [x] implement `fetchPaymentMethods()` and `FetchPaymentMethodsRequest` classes (it's ok if the response is hard coded in the Mollie gateway if there is no API request for this). I want to be able to programmatically discover the available payment methods, and some gateways like Icepay provide an API method for this so we need to be flexible enough for it.