Correct block style
Prevent BufferedSerialDevice#readline ensure block from raising its own exception on cleanup

When reading from @serial_port caused an exception, readline would still try to prepend the nil remaining string, causing an additional error.
Coerce nil to a blank string so that this operation will not mask any error raised within this method.
Add debug logging to the sensor_watch daemon

Closes #25
Allow the daemon to exit gracefully on SIGINT (^C)

Also add a newline anytime we exit with ^C, so the prompt doesn't start with ^C.