[Crab] AI - refactoring, increasing capabilities

by Yurii Chernyi for Battle for Wesnoth

I would like to reorganize the current AI subsystem to allow different people collaborate more efficiently on creating new AIs and improving AIs. Also, I would like to make the Wesnoth AI better. See http://www.wesnoth.org/wiki/SummerOfCodeProposal_AI_Improvement_Crab for all the info.