Rewrite of the Guestbook example in Go

This is a Go version of the Guestbook example. The readme has been ported over from the existing one.

#1271
This line shouldn't be indented (it makes it seem like it's subordinate to the clause above). Same with a couple of places below. In general, I think this would be easier to read if the closing brackets were on a new line and aligned with the clause that opens them, e.g.

html, body {
  background: #EEE;
}