Release v0.1.8

- [x] write release notes
- [x] version bump
- [x] publish to MyGet
- [x] draft release on GitHub
- [x] removed `IGitHubClient.Tree` as it is a duplicate of `IGitHubClient.GitDatabase.Tree`
- [x] test test test
- [x] publish to NuGet
- [x] :shipit: