Move Game factory functions to class methods of Game.

This moves the functions new_tree(), new_table(), read_game(), and
parse_game(), which were previously at the top-level scope of the gambit
package, into classmethods of Game.

The Game class is now exposed in the gambit package, so calls to these
operations should now be written, e.g., gambit.Game.new_tree() instead of
gambit.new_tree()