Added the score reception functionality - method LoadCurrentScore

For the issue #189
Usage example: 
    
    using GooglePlayGames.BasicApi;
    ...
    PlayGamesPlatform.Instance.LoadCurrentScore(  
                               boardID, 
                               LeaderboardConsts.TIME_SPAN_ALL_TIME,
                               LeaderboardConsts.COLLECTION_PUBLIC,
                               (IScore score) => { if(score != null) Debug.Log(score.value); } 
                               );

For now, only works on Android as I have no iOS devices. Sorry. 
Also, there seems to be a bug of unknown etymology - changes to the scores, issued with the ReportScore, are shown immidiately in the Leaderboards UI, but are delayed for approximately a minute when querying using LoadCurrentState.