Added Player class and implemented FlatFileDatabase.add_player.

I added db_exists() to the base Database class to improve readability (particularly with error checking) for each method.

Players are saved in the universe as <name>.player files, there will be a one element of the player class on each line.
Implemented get_player().

This uses a local method of _read_player(), which may be moved to Database if it is useful for other database types.