Add AES block sizes to PGPPacket._cipher_block_size

Add AES block sizes to PGPPacket._cipher_block_size

From the AES spec [1]:

  This standard specifies the Rijndael algorithm (...), a symmetric
  block cipher that can process data blocks of 128 bits, using cipher
  keys with lengths of 128, 192, and 256 bits.

[1]: http://csrc.nist.gov/publications/fips/fips197/fips-197.{ps,pdf}