Added Engine.fortify (previously fortification would break Engine)

Also moved adjacency check from Engine.attack_and_fortify to
Engine.attack and Engine.fortify.  This will make it easier for
subclasses to change the rules (e.g. non-adjacent fortifications).

Also reordered attack / conquer log messages:
  'attacked X from Y' -> 'X attacks Y'
which is shorter and makes the A:B ratios more clear.