[Tutor] Update to my tutorial

[Alan Gauld]

I just spent some time updating my Python tutorial This is not
the big upgrade I discussed a while back, to cater for v2, but
is preparatory to that. It does include:

-  a tweak to the style sheets to help complaints about
   difficulty reading the red code against the blue background.
- many many typos etc fixed thanks to Petr Prikryl one of the
   Czech translators. Hopefully next week I'll get Petr's new
   improved Czech version up too.
- Some code improvements and clarifications requested by other
readers.

In short if you have pointed out an error in the pasy 6 months
it should now be fixed! Any new ones shouldbe notified to me
directly please!

Enjoy,

Alan G
Author of the Learn to Program web tutor
http://www.freenetpages.co.uk/hp/alan.gauld