Stub out PGPPacket._parse_string_to_key_specifier

Stub out PGPPacket._parse_string_to_key_specifier

From RFC 4880 [1]:

       ID          S2K Type
       --          --------
       0           Simple S2K
       1           Salted S2K
       2           Reserved value
       3           Iterated and Salted S2K
       100 to 110  Private/Experimental S2K

In the following sections (3.7.1.1 - 3.7.1.3), the first octet of each
specifier is it's type (simple starts with 0x00, salted starts with
0x01, and iterated and salted starts with 0x03).  That means we can
parse the first octet of the S2K specifier and use its value to
determine which type of specifier we're parsing.

[1]: http://tools.ietf.org/search/rfc4880#section-3.7.1