[#5] Implemented GitHub Integration

- Implemented GitHub API calls:
  - Get files form PR.
  - Get all comments from PR.
  - Create review comment for file in PR.
- Improved the data representation for results (file, rule and item) from records to maps. 
- Updated README.
This file is actually the body of the request the webhook service would receive when a PR is created, but VHS does help for testing the request/responses done when processing the PR event.