New cost_calculator and ai.find_path_with_avoid() function.

Done the task from http://wiki.wesnoth.org/EasyCoding#Improvements_to_AI. "Add a cost function to the C++ path finding code that takes avoided hexes into account".

Added new cost_calculator - shortest_path_calculator_obey_avoid and find_path_obey_avoid functions for ai-context Lua API.

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