Validate form.
        Return True if Django validates the form, the username obeys the parameters, and passwords match.
        Return False otherwise.

def is_valid(self):
        ''' Validate form.
        Return True if Django validates the form, the username obeys the parameters, and passwords match.
        Return False otherwise.
        '''
        if not super(AddUserForm, self).is_valid():
            return False
        first_name = self.cleaned_data['first_name']
        last_name = self.cleaned_data['last_name']
        if User.objects.filter(first_name=first_name, last_name=last_name).count():
            non_field_error = "A profile for {0} {1} already exists with username {2}." \
              .format(first_name, last_name,
                      User.objects.get(first_name=first_name, last_name=last_name).username)
            form_add_error(self, '__all__', non_field_error)
            return False
        if self.cleaned_data['user_password'] != self.cleaned_data['confirm_password']:
            form_add_error(self, 'user_password', "Passwords don't match.")
            form_add_error(self, 'confirm_password', "Passwords don't match.")
            return False
        return True