[#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.
hummmm… do we **really** need macros for these things? Isn't it clearer to write the urls with the ``~_`` things close to the variables that come in as replacements?