Adds `Arrived` behavior for Rooms and implements a basic JS graphic driver (#20)

First, the Room needs to be able to tell the Player what he can see. For now, when the Player sends the `Arrived` command, the Room sends back a list of its children. The Player can then talk to each of them directly to get details. This is simply a temporary implementation. Later, we will use a more Event-driven model of moving into Rooms.

For each of the Room's children, the Player sends his ConnectionService a `create` command. The JavaScript client receives the command and acts according to the command type. In this case, `create` means he should create an object on the `view`. At the moment, the client can respond to `create`, `move`, and `quit` commands.

BUG: possible race condition: When we first start the server and point the browser to it, the `create` command and the subsequent `move` commands work just fine. When I hit `Q` and quit, the server reacts just fine and cleans up resources. When I reload the page, however, I think the Player actor sends the `create` command too quickly, and it hits the browser before it's ready. Consequently, we never see the `create` command, only a bunch of `move` commands. How should we fix this? Should the browser tell the server when it's ready? Or maybe the system should be able to work knowing that some commands will be lost in the network. Either way, it's clear there needs to be more communication.