Support preview Nested Teams API

[gmlewis]

GitHub Announcement:
https://developer.github.com/changes/2017-08-30-preview-nested-teams/

This involves adding new endpoints and changing existing endpoints according to the blog post.

Successful completion and closing of this issue involves addressing all the changes described in the GitHub announcement. Note that it doesn't have to all be done in a single PR. It is always a good idea to carefully document what portions are being addressed in any given PR and if there is still work yet to be done.

Thank you in advance for contributing to this open source project!
Your assistance is greatly appreciated.

[elliott-beach]

I'd like to start working on this.
Currently I'm assigned to #715 but that is quite an insubstantial change.

[dmitshur]

Awesome, thanks @e-beach!

[dmitshur]

As Glenn said, this doesn't have to be resolved in a single PR if that would make it large. If so, it'd be better to break it into a few smaller independent changes, as long as that's possible and doing so makes sense.

[elliott-beach]

I will be creating a NewTeam struct for the params passed to CreateTeam and EditTeam, which now differ from the Team response because of the parent(in response) and parent_team_id(in request) fields. https://developer.github.com/v3/orgs/teams/#parameters

I also noticed that the repo_names parameter to CreateTeam is not supported by go-github currently, so it will also go in that struct.