Added a public_repos method to repos api; added an optional api_url arg to GitHubV3API initializer, so that a Github-Enterprise-style api URL could be passed, e.g., https://git.mycompany.com/api/v3.  Default is still https://api.github.com.
Merge pull request #12 from walquis/ghe-list-all-public-repos

Added a public_repos method to repos api; added an optional api_url arg ...