This CL adds a simple example for StructOf.
The example shows how StructOf can be used in a JSON roundtrip.

Change-Id: I9ff1ea9cb8c0cf297c5fae74e68b89931076adfd
Reviewed-on: https://go-review.googlesource.com/33953
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>