Israel Defense setup change

Since it only fires once anyway, I moved the Defense setup event to a
triggered event and set it fire from the related decision
Exilarch overhaul

They're no longer mercs, are blocked from converting, and some of their
events now properly exclude human players. If there are no issues with
this new setup then the special succession law rules for them can
eventually be removed. They might also need to have the marriage event
adjusted to provide a son.
Merge pull request #46 from thefinestsieve/beta

Exilarch overhaul and Israel Defense tweak