AES example compatible with aesencryption.net

Parameters (must match to interoperate with the site):
  - cipher : AES-{128|192|256}-CBC with PKCS#7 padding
  - key    : raw UTF-8 bytes of the password, zero-padded or
             truncated to the key size (16/24/32 bytes)
  - IV     : the fixed 16-byte ASCII string "12345678b0z2345n"
  - output : Base64

Sample: encrypting "My text to encrypt." with key "encryptor key" (256-bit)
yields:  a8FC/4chLgfmBP1MAM0C/D8KqddSjxepiw6PLI6wlVk=

Run:
  cargo run
