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.

The only thing that needs to be decided is:
- [x] decide on a consistent response format for fetchIssuers() and fetchPaymentMethods(). Right now for example in the TargetPay gateway it returns an associative array, whereas in this gateway you have returned a nested associative array.