* Use the new Tree visitor algorithm
** ASTNode extends VisitedTreeNode
** ASTreeWalker extends TreeWalker
** replace all TreeNodePtr with VisitedTreeNodePtr

* git-ignore 'calc.tab.cpp', 'calc.tab.hpp', 'calc.output'
* ASTNode
** change image from std::string to char array (light weight)
** receiving char array in bison/flex instead of std::string