Fix type of `aiMove`.

The AI needs to know the IDs of the ships. Therefore I added a Fleet argument.
Fixes #22
Some more messages
Implement random ship placement, fixes #25
One more message, better parsing of orientation
Slightly improved layout for placeships
Remove debug output.
Extend AI state for movable ships.
Allow ships to move over already sunk ships.
Draw ships in right order.
Change benchmark to work with new game interface and allow movable ships.
AI: Implement random ship moving.

Given that the AI has no information which positions the user fired
at, I can't think of any better strategy...
Bug fix.

Sometimes, the user's click would select the wrong ship, namely,
when a movable ship happened to be on top of a sunk one.