Add PGPPacket._string_to_key for calculating decryption keys
So far we only implement the 'simple' string-to-key type. From RFC
4880 [1]:
Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session
key size, the high-order (leftmost) octets of the hash are used as
the key.
If the hash size is less than the key size, multiple instances of
the hash context are created -- enough to produce the required key
data. These instances are preloaded with 0, 1, 2, ... octets of
zeros (that is to say, the first instance has no preloading, the
second gets preloaded with 1 octet of zero, the third is preloaded
with two octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets
on the right discarded.
[1]: http://tools.ietf.org/search/rfc4880#section-3.7.1.1