Created the isSusceptibleToAttack method, which is calculated when
generating the list of possible WMs to attack. Currently only has the
submarine entry, however over time it can grow to incorporate new,
special units (e.g. stealth fighter)
-city.rb  was missing a "data/" on on it's images, haven't fully tested
-changed MedTank & Tank classes to have more ammo (although not
officialy set)