Add btime and wtime options

[janrnc]

As issued here, btime and wtime commands are useful in order to let the engine decide the time spent in searching for the best move.

As reported in the UCI description:

If one command is not sent its value should be interpreted as it would not influence the search.

So i thought that the use of optional arguments in the already existing get_best_move function would prevent from having lots of different methods for move searching.

[janrnc]

Sorry for all these fix commits but stockfish output depends on the machine it runs on. Shouldn't we just test this wrapper instead of checking the actual engine output?

Seems that even from one run to another, on the same machine, the output can change and break the tests.

[janrnc]

I've squashed these commits in order to not mess up with the history.

[zhelyabuzhsky]

Thank you :)