Add multiplayer

[jorisvddonk]

Support for multiplayer would be really sweet.

Although socket.io is already used for bidirectional asynchronous communication between server and client, adding multiplayer support could still cost quite a bit of time.

[jorisvddonk]

Started working on this in the 'further-development' branch.

[jorisvddonk]

Some more stuff needs to be done to finish the multiplayer support:

• [x] Bullet hit detection needs to be done somewhere (server-side might be best to prevent cheating, but for now client-side would be easiest to implement. Each client should proabably be responsible for doing hit detection for its own bullets)
• [ ] Bullet spread is currently randomized, but should be randomized based on a seed that's synced amongst clients.
• [x] HP of files should be synced between clients
• [x] Clients need to be put into separate rooms (a room per folder) so that they can't see each other if they're in different rooms
• [x] Dead clients need to be pruned (their ships need to be destroyed from the game)
• [ ] Chat should be added
• [x] Other ships should be displayed on the radar

[jorisvddonk]

To elaborate on the current state of multiplayer support: it was written in about 30 minutes, and works in so far that two players can shoot up files in the game's root folder, but desyncs will eventually happen unless the aforementioned TODOs are resolved.