fix zero time period at control loop start (#30)

diffbot_base/src/diffbot_base.cpp

@@ -25,15 +25,16 @@ int main(int argc, char **argv)
  // Setup for the control loop.
  ros::Time prev_time = ros::Time::now();
  ros::Rate rate(10.0); // 50 Hz rate
-
+ rate.sleep();
+
  // Blocks until shutdown signal recieved
  while (ros::ok())
  {
  // Basic bookkeeping to get the system time in order to compute the control period.
  const ros::Time time = ros::Time::now();
  const ros::Duration period = time - prev_time;
  prev_time = time;
- 
+
  // Execution of the actual control loop.
  diffBot.read(time, period);
  // If needed, its possible to define transmissions in software by calling the