Create an example program demonstrating AES-256/CBC/PkcsPadding encryption and decryption

This is based upon code originally written by @marcusklaas. I have made some changes to add some comments and (hopefully) turn it into a good example. @marcusklaas granted permission to license his code under the terms of the Rust-Crypto licenses (MIT / Apache 2.0) in his comment in #167.

Closes #167