Update version to v19 (google#1056)

Support updates to Marketplace APIs and webhook payload (google#1042)

Helps google#1040.

Restrict request params of CreateHook for repos and orgs to known sub…

…set (google#1018)

Fixes google#1015.

Restrict request params of CreateRelease and EditRelease to known sub…

…set (google#997)

Fixes google#992.

Introduce Go modules and remove support for Go 1.8 (google#985)

Update orgs_members.go (google#871)

Installation: Use *Timestamp instead of *time.Time (google#965)

Migrate IDs to int64 type (google#816)

A helper function "Int64" is also implemented to assist the migration of
unit tests.

Refer google#597.

Merge NestedTeamsPreview branch to master (google#747)

* Add support for fetching parent team (for nested teams). (google#723)

Create mediaTypeNestedTeamsPreview const for
https://developer.github.com/changes/2017-08-30-preview-nested-teams/.

Add parent field to the Team struct.

Set preview API media type in endpoints that return a team, so that
the new parent field gets populated.

Helps google#714.

* Add NewTeam struct. (google#724)

This replaces Team with NewTeam in function that accept a team for
creating or editing a team. The NewTeam struct is similar to the
existing NewPullRequest for creating pull requests.

We need a new struct because the set of parameters for creating or
editing a team contains some parameters that are not in the parameter
for teams, and most properties of teams cannot be used for editing.

The specific change is that Teams are returned with a parent field,
but are created/edited with a parent_id field.

Helps google#714.

* Add NestedTeamsPreview header to remaining endpoints

Fixes google#714.

* Change ParentTeamID field to int (from string). (google#748)

Using the code as written in the `NestedTeamsPreview` branch produces
an error when talking to the GitHub API:

	panic: PATCH https://api.github.com/teams/2514825: 422 Invalid request.

	For 'properties/parent_team_id', "2514824" is not a number or null. []

The documentation at https://developer.github.com/v3/orgs/teams/#create-team
states that this parameter is an id type which appears to mean an integer, not a string.
Also, id is an integer in the sample responses for Get team endpoint, etc.

* Bump libraryVersion

* NestedTeamsPreview: Add the ListChildTeams endpoint (google#758)

Remove ListOptions from RepositoryListAllOptions. (google#751)

The standard pagination options have no effect in the /repositories endpoint,
both empirically and according to the documentation:

> Note: Pagination is powered exclusively by the since parameter. Use the
> Link header to get the URL for the next page of repositories.

GitHub API docs: https://developer.github.com/v3/repos/#list-all-public-repositories

Bump up libraryVersion since this is a breaking API change.