Added a new error, DUPLICATE_NAME.
Modified game_addPlayer. Now, the function checks if the player's name is unique in game.
Added a test which checks if not is added a player which don't has the unique name. Fixed the tests in which were created player with same name.
Now if the player's name not is unique then the player have to insert other name.
Simplified the check a name if is unique.
Merge pull request #208 from EnachescuAlin/issue-182

fix #182