Fixes #34 Contributors: make GitHub username and GitHub repo configurable

- Externalize GitHub repo and authentcaiont to application.conf
- Refactor authenticated GitHub API access since personal access tokens are not supported for organizations
- Configure OAuth client id and client secret in Travis CI config file
- Update README file with new needed GitHub authentication environment variables
Merge pull request #52 from rafacm/issue-34-externalize-github-config

Contributors: make GitHub username and GitHub repo configurable