nacl/box: Add examples

These largely follow the pattern laid out in the secretbox example
file. Hopefully they should demonstrate the best ways to use the `box`
primitives.

Change-Id: I99be5f886e6ffe5f859268d6caa282e263eeacd0
Reviewed-on: https://go-review.googlesource.com/44736
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed