Touch up for uniformity
Touch up for uniformity

- Prefer typed 'URL' over 'String' that is URL
- Mark API as @Preview to communicate that this is subject to change

More branch protection stuff needs to be added. See https://developer.github.com/v3/repos/branches/

stephenc/github-api,jeffnelson/github-api,kohsuke/github-api,recena/github-api,Shredder121/github-api