Create constants for byte messages, implement asbytes so many methods can take Message and key objects directly and split get_string into get_text and get_binary. Also, change int handling to use mpint with a flag whenever the int is greater than 32 bits.
add Message.rewind()
add rewind() method to Message, which just resets the pointer so you can
start reading from the beginning again. this is useful for some tests.
unit tests for Message
spanking new unit tests for Message. i'm trying to fix the embarrassment
of having so little of paramiko testable. next up is Transport!