Add "benchmark" to measure the AI's quality.

The benchmark function runs a given number of games (the AI playing against
itself) and averages the number of shots needed to sink all ships.
(For StupidAI, that's currently around 400...)
Start implementing clever AI. (copy stupid AI)
Output the course of the game when benchmarking.
Implement smarter AI.

Roughly speaking, the AI targets cells where there are more possibilities
for a ship to fit in. In addition, it follows a checkerboard pattern to
reduce the number of shots needed to find all ships.
This way, it needs less then 50 shots on average to sink all ships
(assuming standard rules). So over 60% are hits.
However, this strategy needs to be adapted for movable ships.
Switch to CleverAI.
Fix bug in game logic.

Bug: aiResponse used to be invoked even when it was the player's turn.
Reorganize AI code.
Merge branch 'master' of https://github.com/zrho/afp