Create an enum to handle API errors.

Currently, there's only a function to support GET requests. For clarity, it would be ideal to have a general request function that would get the corresponding method as a param.

Code from #25 needs better documentation.

• Handle auth.
• Build funcs.

Although the code in #25 already has a test included, we should make a better coverage of the code with more tests.

Need to add support for the WebSocket API.

Instead of using get/post/request, generalize over Method::*

Some tests are passing only because there are no transactions. It'd be good to test on mock transactions.

An initial WebSocket implementation was added in #25 but it needs better error handling.

Failure is officially deprecated/unmaintained as reported by the Rust security advisory database.

What crate should we use to replace failure?

Current return is:

Result<(), Box<dyn std::error::Error>>

Ideally, we'd have this:

Result<String, failure::Error>

With the assosiated errors from #1