Manage orgs in GitHub Enterprise

[robbiemcmichael]

Adds the two endpoints for managing orgs in GitHub Enterprise.

Happy to discuss in this PR whether the github library should support GitHub Enterprise features. I've put the first Organizations module under GitHub.Endpoints.Enterprise to try to make it clear that the endpoint isn't supported in the github.com API.

[phadej]

Supporting enterprise endpoints is ok.

We should create a GitHub.Enterprise top-level module re-exporting all enterprise endpoints, so you could get everything just by

import GitHub as GH
import GitHub.Enterpise as GH

[robbiemcmichael]

@phadej Good idea. I've pushed an extra commit, let me know if that's what you had in mind.

[phadej]

@robbiemcmichael that's exactly what I had in mind, thanks!

I'll merge once CI is green.

[robbiemcmichael]

@phadej Is anything else needed here before this one can be merged?

[phadej]

A ping, clearly :)