Add secret-key decryption to _parse_generic_secret_key_packet

Add secret-key decryption to _parse_generic_secret_key_packet

The checksum checks will fail with encrypted algorithm-specific key
data.  This adds handling to decrypt the encrypted section with a new
PGPPacket.decrypt_symmetric_encryption stub that still needs to be
filled in.  From RFC 4880 [1]:

   With V4 keys, a simpler method is used.  All secret MPI values are
   encrypted in CFB mode, including the MPI bitcount prefix.

   The two-octet checksum that follows the algorithm-specific portion
   is the algebraic sum, mod 65536, of the plaintext of all the
   algorithm- specific octets (including MPI prefix and data).  With
   V3 keys, the checksum is stored in the clear.  With V4 keys, the
   checksum is encrypted like the algorithm-specific data.

[1]: http://tools.ietf.org/search/rfc4880#section-5.5.3