worker: add isatty to the MessageWriter in Disco.

Now if someone calls isatty() in a job, disco returns False.
This closes issue 351.
ddfs chunk: add a notes to the help message of ddfs chunk.

The help message describes usage of ./ for local files and the use of - for
chunking from stdin.
This closes issue #293.
Merge pull request #532 from pooya/small_fixes

Small fixes