crypto/cipher: Add AES-GCM encryption and decryption example

crypto/cipher: Add AES-GCM encryption and decryption example

Add example of how to use the aes package to
implement AES encryption and decryption
within an application.

Per feedback, use more secure AES-GCM implementation as an
example in crypto/cipher instead of AES directly.

Change-Id: I84453ebb18e0bc79344a24171a031ec0d7ccec2e
Reviewed-on: https://go-review.googlesource.com/18803
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>