empires: Rename add_player to register_player

empires: Rename add_player to register_player

We still haven't yet ported the Empires implementation to play nicely
with all of the new functionality in the base Game class. In
particular, Game now has a base "add_player" method with different
semantics than the older "add_player" method in Empires.

To avoid Empires.add_player from being inadvertently called, we rename
it here to register_player.

This fixes, for the moment, a failure when trying to create a new
Empires game of:

    TypeError: player.remove_connection is not a function