Improve support for OAuth for GitHub

- Add an optional scope parameter to the services oauth method
- Add a scope to the GitHub integration
- Add the omniauth-github gem to the gemspec
Put the GitHub integration under TrepScore::Services
Improve the error for when a repo is not found
Require omniauth-github
Don't require the fields provided by OAuth
Merge pull request #14 from 25-ventures/feature/oauth

Feature/oauth