Implement Meta endpoint

[PauloMigAlmeida]

Before submitting a PR:

We love getting PRs, but we hate asking people for the same basic changes every time.

• [] Push your changes to a branch other than master.
• Add JavaDocs and other comments
• Write tests that run and pass in CI. See CONTRIBUTING.md for details on how to capture snapshot data.
• Run mvn install locally. This may reformat your code, commit those changes.

I was working on #582 when I realised that the /meta endpoint wasn't available in this library.
I created the Javadocs, tests and removed some unused imports on GitHubTest

[bitwiseman]

@PauloMigAlmeida
I cleaned up GHMeta (7d1e977) and then added some examples of different ways to create data objects (7e05ce3) to explain some of the options and reasoning.