get order by order id or custom variable

Enables getting an order by a custom variable or the order id that was generated by Coinbase. 

This is useful for reconciliation of Coinbase's system with an internal one, checking if the statuses match. 

This includes a test that worked against the production Coinbase API.

Also included are some refactorings to support GET and also checking errors, which could be a single error or an array, from testing I saw this.  
I'm a bit worried having two Error properties in the response object. Semantically it doesn't feel right. I'd like to investigate a bit more to see if there's any way possible to keep Coinbase's API smell from leaking into the C# API.

Also, Is the Error property needed to parse error responses resulting from an Order's call? I'll merge this code into pr/8 branch. Thanks very much @ryanmwilliams .