OBJExporter: handle children objects

+ scale factor param
OBJExporter: convert indents (spaces to tabs)
OBJExporter: remove useless check for object.children
OBJExporter: remove scaleFactor
OBJExporter: use object tag instead of group tag for object declaration
OBJExporter: move index vars in parse function
+ use object#traverse to parse each children
Merge pull request #5457 from Colmea/dev

OBJExporter: handle children objects