+ support ROS_LOG_DIR

Hi atiderko,

I just started playing around with your multimaster-package and made a couple of small changes/additions, that might be of interest to you (see this and the following pull-requests).

+ added support for the ros log directory being set through the ROS_LOG_DIR environment variable (see http://www.ros.org/wiki/ROS/EnvironmentVariables#ROS_LOG_DIR)

Thanks & cheers, M.