Allow short algebraic notation

[martsberger]

It would be nice if we could input the moves using the short algebraic notation that is standard in the pgn format, e.g., e4 e5 Nf3, in addition to the long format currently supported.

The public api would get only one additional method, Game.apply_san_move(san_string). I have already implemented this and will open a pull request. I'd like to use Chessnut in a hobby project, but want to be able to read in pgn games.

[ctataryn]

@cgearhart would be great if you could take a look at this and merge if approved.