Use a pty when running interactively with one worker

Running something with only one worker usually means that I want to do
this interactively. Thus, I need a tty (proper behaviour for prompts
etc.).
bw run: Use line buffer and LOG.* only for "workers > 1"

Fabric actually does a pretty good job regarding interactive
output/input. The big problem is LOG: It always prints *lines*. For
interactive stuff like prompts, this won't work.

So, when running with one worker, turn off our own line buffering.
Fabric then uses sys.stdout/stderr directly.
LifeBuffer should be passed just as stdout/err is

there is no reason to bury it three calls down and pass an extra parameter all that way
have output format match that of Fabric

I don't like having to do this.
get rid of _format_output since we're just using recreating Fabric's output anyway
Merge branch 'run-direct'

Closes #10.