Add support for creating RMAs

[samacs]

We're currently using this project to synchronize product images and stock.

We are planning to implement the creation of return fulfillments but there's no support for a couple of methods needed to create this.

• ListReturnReasonCodes - We are working with different channels not just for Amazon, and this method is useful for validating the reasons a client is trying to return an item.

• CreateFulfillmentReturn - This is the method that actually creates the RMA on Amazon's end. Further calls with the same SellerFulfillmentOrderId will expose the a status in order to track the fulfillment.

I'm working in the implementation of these two methods already, just wanted to create the issue before creating the PR.

[hakanensari]

@samacs, yes, these two operations are missing in the client. I don't know if this was originally an oversight or whether Amazon silently added them recently. Either way, yes, we should incorporate them, so :+1: for a PR.

[samacs]

PR #93 is ready!