defense changes

There have been 3 new changes to the Scratch RPS (and thus also the Squawk RPG,) related to defense:

• First and most importantly, the core rolling mechanism is that instead of trying to roll over a difficulty level to succeed, we now have a roll (plus ability bonus) instead equal to a difficulty in order to succeed. This means that defense is now what your attack roll + ability bonus has to be in order to successfully attack that target (equal to or greater than the target's defense.)
  
• Second, we are now again listing character's defense explicitly (after tgh/2.)
  
• Third, all healing rolls now have a difficulty level of 10 (they used to vary more than this.)