[#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.
Even when we can use maps, I would keep the building (and internal structure) of the results **inside** ``elvis_result``