Reworked peg_peg example to align with the previous changes.

Rules that consists of just a crossref to another rules were resolving
as a terminal rule down the chain. This fix will make a new rule as
a clone of the last rule down the chain but under a original rule name.

Parsing expressions keep track of the previous parsing expression during
recursive descent. This enables StrMatch to mark terminal for
suppression if called from Sequence.

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.