Barbarian ship disappeared too fast

[fire-eggs]

1. get 'barbarian invasion' warning at turn x; barbarians don't land
2. get 'barbarian invasion' warning at turn x+1; barbarians land
3. barbarian ship disappears

A: don't double-warn: barbarians should land at turn x B: barbarian ship should not disappear until turn AFTER barbarians land

[fire-eggs]

RE: item A: see issue #103

RE: item B: In AI.Barbarians.cs, BarbarianMoveWater, the first check is to see if the ship is empty, and if it is, the ship is destroyed. This logic may take effect the same turn as when the units on the ship disembark.

I think a "passengers disembarked" tracker is needed, set to the turn # all units disembarked. Until that turn # is exceeded, the ship should not disband.