list organizations

It'd be nice if we could list all orgs using this api. https://developer.github.com/changes/2015-06-17-organizations-endpoint/

```javascript
let gh = new GitHub();
gh.listOrgs().then(...);
```

Start with the linked GitHub Organizations endpoint documentation and search the repository for existing list methods and the GitHub class entry point. Implement and expose the requested listOrgs() operation following the surrounding API conventions, then verify that it returns all organizations as described by the endpoint.

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