(RE): New constant.

(EOF): New constant.
(TIMEOUT): New constant.
(CONT): New constant.
(BadArgs): New exception.
(_expect_before): New variable.
(_expect_after): New variable.
(expectable.fill_buffer): Treat EIO as end-of-file.  You apparently
	get this from a pty when the child exits, at least on Linux.
(spawn.send): Log what is sent via debug.
(impl.loop): Implement expect_before and expect_after.
(_expect_before): Removed.
(_expect_after): Removed.
(impl.__expect_before): New method.
(impl.__expect_after): New method.
(impl.__run_expectations): New method.
(expect_after): New function.
(expect_before): New function.
(__validate_expectations): New function.