Nuget package updates and order refund

[ryanmwilliams]

Nuget packages are updated.

There is also the ability to refund an existing order and send the refunded amount to an address with a certain currency. I tested this with an integration test against the real Coinbase API using real info.

[bchavez]

Hi @ryanmwilliams

Thanks for the pull request.

I've merged your PR, but there were some changes/refactorings I made:

• TotalAmount class was a duplicate of Price in the existing object model.
• OrderRefundResponseDetails looked very similar to Order with a few extra fields, so just sublcassed Order via RefundOrder with the additional refund fields.
• Added support for Mispayments and other options.
• Changed the CoinbaseApi.Refund() signature for better API semantics.

Could you do another refund test to ensure everything is still working? Thanks!

If you can confirm the refund integration test works, I can do a push to Nuget. Otherwise, I'll let this merge sit in master for a bit for more reviewing.

Thanks, Brian