gazebo_plugins: Added updateRate parameter to the gazebo_ros_imu plugin

This PR adds the `<updateRate>` parameter to the `gazebo_ros_imu` plugin. I added it because at the moment the plugin runs as fast as possible.

Usage example:
```xml
<gazebo>
  <plugin name="imu_sensor" filename="libgazebo_ros_imu.so">
    <updateRate>100.0</updateRate>
    <topicName>imu_sensor</topicName>
    <bodyName>IMU_LINK</bodyName>
  </plugin>
</gazebo>
```
Style comment - need a couple other spaces "(1.0 / this->update_rate_)"