added visit_topdown methods to Visitor classes

Copy exc state when converting UnexpectedCharacters to UnexpectedToken exc. Fixes #462

Fixed some typos in the docs

Fix SyntaxError with Python 3 in JSON tutorial

unpacking a tuple argument is invalid in Python 3.

    >>> def foo(a, (b, c)):
      File "<stdin>", line 1
        def foo(a, (b, c)):
                   ^
    SyntaxError: invalid syntax

Fixes #403

I (erez) rebased the 'master' branch due to a special circumstance, inolving one of the contributors.
I really hope that won't happen again. If it broke your build or patch, I sincerely apologize, and please let me know.

Rule.order should be set as the index of each expansion with rules
of the same name (e.g: a : b    # rule.order 1 | c    # rule.order 2).