Update Italian translation

Hi Jeroen,

updating the translation I edited also 'help-en.html' and 'help-it.html' to add my name as in 'about-en.html'.
Considering that the new strings are put after 'Chess is on GitHub - contribute and make it a better app!', I hope others can be tempted to contribute with further translations. Or, maybe it is only vanity :)

Regards
  Paolo