Update README.md, fix the license name in setup.py

[skupriienko]

Actions:

• Fix the api_credential URL
• Use api_key and api_secret lowercased consistently
• Use print() for Python 3 support with a suggestion Python 2 will be dropped in the next PR
• Update an example in Make your first call to fix README is outdated #89
• Fix the API Reference URL
• Use id_ instead of builtin id
• Fix the license name in setup.py

[DanyilNefodov]

LGTM overall