Added fortification to the basic Player AI.

Added fortification to the basic Player AI.

This will excercise the fortification code during random_game()s.

Also moved Player.border_territories functionality into a combo of
Player.territories and a new Territory.border.  This makes it easy to
iterate through non-border territories looking for out-of-the-way
armies for fortification.

Finally, limit the players to one fortification per turn ;).