support and document multiple peer connections

[cvan]

Right now, we fire a busy event with the following error:

Pair code `${pairCode}` already in use

We should be able to connect to a small number of peers (à la https://github.com/feross/simple-peer#connecting-more-than-2-peers). We should also document it.

Thoughts, @potch?

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Related projects:

• https://github.com/haydenjameslee/networked-aframe — view demos
• https://github.com/mikeal/roll-call — view demo
• https://github.com/mikeal/killa-beez
• https://github.com/mikeal/cuban-linx/tree/master/lib
• https://github.com/mikeal/signal-exchange
• https://github.com/mikeal/bong-bong
• https://github.com/mikeal/d-block/tree/master/lib
• https://github.com/mafintosh/webrtc-swarm