turn .publish() into a normal async function returning a promise.

this simplifies the code and makes the API more intuitive.

we used to need the event emitter thing because we were subscribing to the same relay
to check if the event had been published, but that is not necessary now that we assume
an OK response will always come.

closes https://github.com/nbd-wtf/nostr-tools/issues/262

README.md

@@ -108,13 +108,7 @@ let event = {
 event.id = getEventHash(event)
 event.sig = getSignature(event, sk)
 
-let pub = relay.publish(event)
-pub.on('ok', () => {
-  console.log(`${relay.url} has accepted our event`)
-})
-pub.on('failed', reason => {
-  console.log(`failed to publish to ${relay.url}: ${reason}`)
-})
+await relay.publish(event)
 
 let events = await relay.list([{kinds: [0, 1]}])
 let event = await relay.get({