MkGithub mock version of Github server

This version introduces class MkGithub, which is a mock version of Github server-side implementation. This is how you can use it in your unit tests:

Github github = new MkGithub();
Repo repo = github.repos().create(
  Json.createObjectBuilder().add("name", "test").build()
);
Issue issue = repo.issues().create("title of the issue", "body of it");
issue.comments().post("How are you?");