No Spam class: Prevent robot spamming through web forms

This class can be used to make it difficult for robots to write spam in forums, contact forms etc. It uses a form parameter with a 'random' hash that is used in both the form page and de processing page. It can be used in links and forms.
The links or forms that process the hashed parameters can verify the hash and only allow to proceed if the hash of the current date matches the followed URL hash parameter. This may help preventing that robots to spam forums, guestbooks and blogs.