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
I'm not sure we have any sort of project-wide javascript style that we are following, but the existing files seem to have a style that is more like https://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml than this file does. 

@brendanburns @jbeda Do you have a preference for javascript style that we should shoot for?