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.