Peer should be an EventTarget and emit events

[sgwilym]

What's the problem you want solved?

There is no way to observe when certain changes have taken place to a Peer without being directly responsible for those changes yourself.

Is there a solution you'd like to recommend?

Peer should extend EventTarget and should be able to pass on events from its underlying Auth (#341), and for when a new share becomes available thanks to the addition of a capability.