Correct velocity units with the # of motor pole pairs

[ssr-yuki]

closes #18 and #20

• corrects velocity units by parameterization of the number of motor pole pairs
• renames variables and functions with RPM to nemes with ERPM
• enables ROS interfaces to use SI units, i.e. [rad/s]

For sample execution, please execute the following command:

roslaunch vesc_driver vesc_driver_node.launch