crypto/des: add an example to demonstrate EDE2 operation.
crypto/des: add an example to demonstrate EDE2 operation.

EDE2 is a rare DES mode that can be implemented with crypto/des, but
it's somewhat non-obvious so this CL adds an example of doing so.

Fixes issue 3537.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/6721056
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go