roslaunch option to log to both: screen and logfile

Fix for issue #409

I added a new ouput value `both` which sends stdout and stderr to both screen and file. it uses subprocess.pipe and threading.

Also includes the following fix:

on http://wiki.ros.org/roslaunch/XML/node#Attributes it says that:

If 'log', the stdout/stderr output will be sent to a log file in $ROS_HOME/log, and stderr will continue to be sent to screen.

but stderr was only sent to screen, not to logfile. now stderr will be sent to both screen and log, while stdout only goes to log.