nacl/secretbox: add license header and package prefix

Name the example package "secretbox_test" so we use the public
interface for secretbox in examples. This way the code appears the
same way a user would have to type it.

Change-Id: I0487e39927ddae297ea45e09a3fa397779a8e21d
Reviewed-on: https://go-review.googlesource.com/28222
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed