Add a feature to build a cost_map in pathfind.xpp

In a cost_map each hex is mapped to total cost to reach this hex
(for one or multiple units).
Note that the runtime of find_reach() is not really affected
when the parameter full_cost_map is NULL (default).
Add lua function find_cost_map()

Temporarily see http://pastebin.com/PbmYvW6S
Later a wiki entry is added on http://wiki.wesnoth.org/LuaWML:Pathfinder