Add iterated-and-salted string-to-key support to PGPPacket._string_to_key

Add iterated-and-salted string-to-key support to PGPPacket._string_to_key

From RFC 4880 [1]:

  Iterated-Salted S2K hashes the passphrase and salt data multiple
  times.  The total number of octets to be hashed is specified in the
  encoded count in the S2K specifier.  Note that the resulting count
  value is an octet count of how many octets will be hashed, not an
  iteration count.

  Initially, one or more hash contexts are set up as with the other
  S2K algorithms, depending on how many octets of key data are needed.
  Then the salt, followed by the passphrase data, is repeatedly hashed
  until the number of octets specified by the octet count has been
  hashed.  The one exception is that if the octet count is less than
  the size of the salt plus passphrase, the full salt plus passphrase
  will be hashed even though that is greater than the octet count.
  After the hashing is done, the data is unloaded from the hash
  context(s) as with the other S2K algorithms.

[1]: http://tools.ietf.org/search/rfc4880#section-3.7.1.3