Sanitizes data so that Cross Site Scripting Hacks can be
prevented. This method does a fair amount of work but
it is extremely thorough, designed to prevent even the
most obscure XSS attempts. Nothing is ever 100% foolproof,
of course, but I haven't been able to get anything passed
the filter.
Note: Should only be used to deal with data upon submission.
It's not something that should be used for general
runtime processing.