Added Global Achievements and Player Achievements

Moved cache for GameStatsSchema to ISteamUserStats

Allow UserAchievement object to be used by either getPlayerAchievements
or getUserStatsForGame

Added steamId to UserStats

Added functionality for getGlobalAchievementPercentagesForApp,
getNumberOfCurrentPlayers and getPlayerAchievements

Removed getGlobalStatsForGame as it is not clear how to call it
correctly.

Added unit tests and supporting json