[mod] handle indentation when previous is None
[mod] handle .indentation for inner node
[mod] finally, in 'if a: pass' pass.indentation == '' instead of '    '
[mod] make comment more readable
[mod] refactor, do all the same things in the same place to avoid confusion
[enh] handle all syntaxe cases
[enh] add utils folder
[enh] add indentation_test.py in utils folder