Fixed bug in parser model construction from python grammar description
with direct rule call.

Converting terminal to a string as a default action.

SemanticAction now have a default behaviour which will reduce overhead
in some common cases.

Arpeggio now supports both python 2 and 3.

This works for rules that are referenced from the rule which created
NonTerminal.

Introduced grammar rules decorator. Combine decorator is used to
mark part of the grammar as lexical rule. This rule will always
return Terminal, whitespaces will be preserved, and comments will
not get matched.