copied the usage example from the documentation over here
fixed new-style packet header length encoding
fixed an error with parsing new-style packet header lengths
set defaults for some convenience in creating new packets
refactored the functionality of PGPPacket into class Packet, and used that to clean up the init methods of Packet subclasses
used new Packet init and Header defaults to clean this up a bit
not sure this test is useful - commented for now
added to the signature filename generated by test_sign so that I can pgpdump the generated signatures for debugging just by commenting out line 169