Better grammar.txt

[MartinRinehart at gmail.com]

I previously posted a link to an improved, HTML-based, hyperlinked
grammar.txt. Discard that one. This one is much better.

http://www.martinrinehart.com/articles/python-grammar.html

If you find it useful, thank Gabriel Genellina for encouraging me to
get it really right.

It includes three corrections to grammar.txt (imagnumber, xor_expr and
and_expr) that I've reported.