add key exchange tests + 1 more sftp test
add test suite for key-exchange protocols, since i apparently broke the
"gex" protocol recently and never noticed. also add an sftp unit test for
mkdir/rmdir.
fix test.py to use options instead of env vars, sftp tests default off
fix up the test framework so that the sftp unit tests aren't always run (you
have to ask for them explicitly) and they take their configuration from
command-line options. they still require a remote server.
add unit tests
add unit tests for BufferedFile and SFTP (it's a start). remove the demo sftp
client because it was 99% copied from the other demos, which makes it kinda
confusing. the unit tests are a much better example.
Robey Pointer