Removed debug print
fix #15 Node visitor implementation.

Implemented node visitor pattern.
Idea suggested by Yash in issue #15.
calc_peg_visitor.py example shows basic usage.
PEG modules refactored to use visitor pattern in semantic analysis
Small change in calc_peg example
peg_peg example reworked to use PEGVisitor
Reworked bibtex example to use visitor.
Reworked robot examples to use visitor
Fixed bug in visit method
Visitor test
Merge branch 'visitor'