more unit tests
add a unit test for sending a large (1MB) file with line buffering but no
linefeeds (this triggered several bugs and inefficiencies), and another test
to verify that the write buffer is flushed on seek.
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.