Add GitHub.API.Gateway which calls HTTPoison

[joshsmith]

Problem

We should add an API.Gateway which simply has a request method that calls HTTPosion. Response parsing can be done in this module. We can then mock the gateway in tests, and mock %HTTPoison.Response{} so that we can include the headers (e.g. for pagination).

We'll need to refactor GitHub.API.EagerAPI to make use of this in marshall_response

• check if the headers contains a "Links" key
• if it does, it calls the code which makes additional requests for further pages

[joshsmith]

Here's an example of mocking HTTPoison https://hashrocket.com/blog/posts/mocking-requests-in-elixir-tests