Implement Thief Ambush Merit ability

Checks for the ambush trait and that the player is behind the monster, the same as it checks for sneak attack.  Each level of the merit should increase accuracy by 3.  Also added this to the ranged check as a final accuracy boost for both throwing and ranged weapons.
Fixed typo in comment
Fixed missing '))'

In the ranged accuracy check
Merge pull request #280 from towbes/Ambush

Implement Thief Ambush Ability