Add support for GitHub Integrations

[talex5]

Not out of beta yet, but would be great to have support for this:

https://developer.github.com/changes/2016-09-14-Integrations-Early-Access/

For DataKitCI, we'd like to be able to:

• Clone/pull from the repository.
• Set statuses on commits.
• Be notified when PRs, refs or status change.
• Not have to register web-hooks individually on each repository we monitor.

Currently, we have to ask users to give us read/write on all their repositories, which is pretty bad.

[XVilka]

Just for the record - Integrations were deprecated. Now the recommended way is to create applications using the GitHub v4 API (GraphQL). See more at:

• https://developer.github.com/v4/
• https://graphql.org/
• https://developer.github.com/marketplace/getting-started/
• https://developer.github.com/v4/guides/migrating-from-rest/

It will require GraphQL library, e.g. graphql. See its repository andreas/ocaml-graphql-server