Stockfish

* added method `set_depth` to allow user to change depth used in `get_best_move`
* added method `get_evaluation` to get current score of the game in centipawns or mate in x

Thank you, 2kofawsome

* Improved README
* WARNING: removed argument `moves` from `get_fen_position` method

* added method `get_fen_position` to return FEN notation of current board's position
* added method `show_board_visual` to print a visual representation of current board's position

Thank you, VivekThazhathattil

* Added resetting of info after setting position

* Added clearing info after setting fen position (thanks for charlesoblack for bug report)

* WARNING: parameter `params` in Stockfish's constructor renamed to `parameters`
* Added method `get_parameters` to get current engine's parameters
* Method `set_skill_level` also changes engine's parameters attribute
* Other small improvements and fixes