Add and test cmd_vel_mux to default navigation launch file

[jrcapriles]

If we are going to have multiple sources of cmd_vel, it would be better to have somebody arbitrating the topics using priorities. cmd_vel_mux seems to offer this capabilities.

[griswaldbrooks]

The motivation for this is use of the android app, while the robot is driving.
Because it's not trivial to get the android app to publish something other than cmd_vel, the idea is that we will have move_base publish something like cmd_vel_move_base and have the Teensy subscribe to a topic like cmd_vel_mux which the cmd_vel_mux package would publish.

[griswaldbrooks]

This looks like a better candidate.
http://wiki.ros.org/yocs_cmd_vel_mux