Create CvGame.h

getCurrentHighestEra added
Create CvGame.cpp

GetCurrentHighestEra added
Known civ bonus based on most advanced player era

Known civ bonus is changed so that it is based on the most advanced player era with following equation:
 HighestPlayerEra*TECH_COST_TOTAL_KNOWN_TEAM_MODIFIER
 0 = ERA_ANCIENT
 1 = ERA_CLASSICAL
 2 = ERA_MEDIEVAL
 3 = ERA_RENAISSANCE
 4 = ERA_INDUSTRIAL
 5 = ERA_MODERN
 6 = ERA_FUTURE
Max player count 40

Max player count increased to 40