Add PGPPacket._old_format_packet_lengths

Add PGPPacket._old_format_packet_lengths

From RFC 4880 [1]:

   The meaning of the length-type in old format packets is:

   0 - The packet has a one-octet length.  The header is 2 octets long.

   1 - The packet has a two-octet length.  The header is 3 octets long.

   2 - The packet has a four-octet length.  The header is 5 octets long.

   3 - The packet is of indeterminate length.  The header is 1 octet
       long, and the implementation must determine how long the packet
       is.  If the packet is in a file, this means that the packet
       extends until the end of the file.  In general, an implementation
       SHOULD NOT use indeterminate-length packets except where the end
       of the data will be clear from the context, and even then it is
       better to use a definite length, or a new format header.  The new
       format headers described below have a mechanism for precisely
       encoding data of indeterminate length.

The struct format characters are [2]:

  Format   C Type           Python type   Standard size   Notes
  B        unsigned char    integer       1               (3)
  H        unsigned short   integer       2               (3)
  I        unsigned int     integer       4               (3)

  3. When attempting to pack a non-integer using any of the integer
     conversion codes, if the non-integer has a __index__() method
     then that method is called to convert the argument to an integer
     before packing.

     Changed in version 3.2: Use of the __index__() method for
     non-integers is new in 3.2.

[1]: http://tools.ietf.org/search/rfc4880#section-4.2.1
[2]: http://docs.python.org/3/library/struct.html#format-characters