Now supports entire Python 2.0 language and still supports Python

Now supports entire Python 2.0 language and still supports Python
1.5.2.  The compiler generates code for the version of the interpreter
it is run under.

ast.py:
    Print and Printnl add dest attr for extended print
    new node AugAssign for augmented assignments
    new nodes ListComp, ListCompFor, and ListCompIf for list
        comprehensions

pyassem.py:
    add work around for string-Unicode comparison raising UnicodeError
        on comparison of two objects in code object's const table

pycodegen.py:
    define VERSION, the Python major version number
    get magic number using imp.get_magic() instead of hard coding
    implement list comprehensions, extended print, and augmented
        assignment; augmented assignment uses Delegator classes (see
        doc string)
    fix import and tuple unpacking for 1.5.2

transformer.py:
    various changes to support new 2.0 grammar and old 1.5 grammar
    add debug_tree helper than converts and symbol and token numbers
    to their names