Created a new parameter to move_base: 'num_spinner_threads'.

Depending upon the value passed to this new parameter, move_base will start up
and run with a single-threaded spinner (current behavior) or if passed a value
of 2 (threads) or greater, move_base will start a multi-threaded spinner (using
ros::AsyncSpinner) with the desired number of threads.

This behavior was created to address a latency issue that is described here:
http://answers.ros.org/question/148171/latency-when-sending-goal-to-move_base-and-it-starting/

I'm not sure of your testing procedure for integrating changes into the project nor move_base's overall thread safety. None-the-less, I thought I'd pass this along to you for review. I'm more than happy to follow proper protocol to get this patch (or similar) integrated to the master in order to have this functionality available in the core ROS move_base.  Thanks, Tom.