Handle the entire API and forward it to the Handler
Implemented: Handler#update_position
Implemented: Handler#get_current_game_state
Implemented: Handler#finish_game
Refactored: Handler
Improved: Handler#finish_game
Implemented: Handler#get_game_statistics
Implemented: Handler#set_player_statistics
Refactored: Handler
Cleaned-up: HandlerSpec reorganized and consistent naming

* Organized in two groups depending on the communication partner
* Consistent naming of spec names and raise_exception (not raise_error)
Refactored: HandlerSpec
Implemented: Handler#get_game
Implemented: Handler#create_lobby
Implemented: Handler#create_player
Implemented: Handler#join_lobby
Implemented: Handler#list_lobbies
Implemented: Handler#leave_lobby
Cleaned-Up: Handler
Refactored: HandlerSpec's "should return self" specs
Extracted the knowledge about the Lobby class from Handler