Refactor github/__main__.py to conform with the style of the rest of the project

[vivus-ignis]

What would you like to be added:

• OOP style for the code in github module

Why is this needed:

Suggestions from @NotTheEvilOne:

• do not use s3_artifacts._bucket property, use bucket (without underscore) instead
• add support for semantic versioning
• use commit_id parameter for CName constructor
• make github module and FlavorsParser behavior consistent with regards of what flavors.yaml they're using