Fix build of example/echo.hs.

In the "echo" example, 1) don't respond to error messages and 2) allow
subscriptions.

In the "echo" example, send a ping every 60 seconds to prevent the server from timing out the connection.

Add a note that servers populate the "from" attribute on stanzas, in the echo example.

Add an example XMPP application, which echos messages back to their senders.