Prevent potential errors with the SF process, and misc.

[johndoknjas]

This PR has a number of small things:

• If the user calls the __del__ method themselves, this could potentially result in an error, since the OS may call __del__ later. So in each call to __del__, ensure that self._stockfish is still running (i.e., = None).
• A check is also implemented in the _put function, such that if self._stockfish isn't running, or if the "quit" command has already been sent to Stockfish, then the stdin.write() and stdin.flush() lines won't be executed.
• Renamed the self.stockfish instance variable to self._stockfish, in order to indicate that it should be treated as private.
• Changed the default search depth from 2 to 15, in case the user doesn't set it themselves. 15 is a good depth value since it's nearly instantaneous while still being fairly strong.
• Added to the example in the readme for creating a Stockfish instance, in order to show how to send all the optional arguments together.

[zhelyabuzhsky]

Good job 👍🏻