Sign timestamp values

This makes sense to me; I think I had an internal TODO to look into doing this way back when I was first building the module... and completely forgot about it. The 7.x patch looks good, but I also need to get it into 8.x first, so would you (or anyone else) be willing to work on the port soon? I'll try to get to it soon, but this summer will be a little busy for me.

@Heine - This works great! I've made one slight change, using a default $honeypot_time value of 0 (int) instead of FALSE (bool), so the function always returns an int. It works just the same with the validation logic, but makes one fewer function in the world with mixed return values :)

Next up: porting to D8.

Looks like we lost a "!" in "!$honeypot_time" on commit for the 8.x-1.x branch.

On commit 0e1ef2c: (not on the current patch)

   // Make sure current time - (time_limit + form time value) is greater than 0.
   // If not, throw an error.
-  if (time() < ($honeypot_time + $time_limit)) {
+  if ($honeypot_time || time() < ($honeypot_time + $time_limit)) {
     _honeypot_log($form_state->getValue('form_id'), 'honeypot_time');
     $time_limit = honeypot_get_time_limit();

7.x-1.x branch looks fine