split out Packetizer, fix banner detection bug, new unit test
split out a chunk of BaseTransport into a Packetizer class, which handles
the in/out packet data, ciphers, etc. it didn't make the code any smaller
(transport.py is still close to 1500 lines, which is awful) but it did split
out a coherent chunk of functionality into a discrete unit.
in the process, fixed a bug that alain spineux pointed out: the banner
check was too forgiving and would block forever waiting for an SSH banner.
now it waits 5 seconds for the first line, and 2 seconds for each subsequent
line, before giving up.
added a unit test to test keepalive, since i wasn't sure that was still
working after pulling out Packetizer.