Go-github: Support additional endpoints for GitHub Apps API

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2018-04-17-additional-endpoints-for-github-apps/

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue, and we can send you an invite to contribute to the repo (which you then accept after you enable two-factor authentication) and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[xabxx]

@gmlewis could you send me an invite? would like to be assigned to this issue. thanks!

[gmlewis]

It looks like I'm currently unable to send invites. @willnorris - could you please send @xabxx an invite and also assign this issue to @xabxx ? Thanks!

[xabxx]

@gmlewis @willnorris hey guys, any updates?:)

[gmlewis]

Hi @xabxx, I recommend just going ahead to start working on it... don't let the assignment hold you up. It is pretty clear from this thread that you are planning on working on it, so others shouldn't start work until they check here on this thread. :grin:

[gmlewis]

Updated GitHub Developer API announcement for this issue:
https://developer.github.com/changes/2018-04-25-new-user-to-server-endpoints-for-github-apps/

[gmlewis]

Another updated GitHub Developer API announcement for this issue:
https://developer.github.com/changes/2018-05-21-additional-endpoints-for-github-apps/

[waseem18]

As there's no PR for this yet, I'm interested in taking up this issue.

Edit: I see a PR has been merged and it resolves this issue.

[gmlewis]

Thank you, @waseem18. It looks to me like you are correct.
I'm going to close this issue.

If anyone finds anything missing, please re-open and identify which one(s) are missing.