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
That's OK with me as long as we can still build the entire codebase recursively (godeps go build ./...) which right now isn't possible. If you move the code into a directory that begins with an underscore, it won't be included in the recursive build and we can keep the external dependencies without worrying about vendoring.