Add MUC echo example

When first getting used to the XMPP protocol and blather I was having trouble getting an MUC bot working. Example would have been helpful, so I wrote one up.
This conditional can be moved into the guards as so:

```ruby
message :groupchat?, :body, proc { |m| m.from != jid.stripped }, delay: nil do |m|
```

That ugly proc would be cleaned up by #135 and `delay: nil` should use a future opposite counterpart to `Message#delayed?`, maybe `Message#now?`.