Contract Killing Changes

Made some changes to Contract Killing:
-Mercenary HQ requires less hammers, and provides a slight espionage
point bonus (hopefully with these changes there is more incentive to
build the HQ).
-Added a new unit called Masked Mercenary that can attack without
declaring war.

Future Plans:
-Mercenaries become obsolete with Gunpowder, and become replaced by new
mercenary units (with firearms instead of swords).