Add support for battery state

[jacobperron]

Add ROS level API for the following libcreate functions:

uint16_t getVoltage() const;
uint16_t getCurrent() const;
uint8_t getTemperature() const;
uint16_t getBatteryCharge() const;
uint16_t getBatteryCapacity() const;

[jacobperron]

#9 Done.

[jacobperron]

Although I like having the various information regarding the battery state as individual topics, it might make more sense to bundle them into the existing sensor_msgs/BatteryState message.

[jacobperron]

Could use the sensor_msgs/Temperature message for the battery temperature since sensor_msgs/BatteryState doesn't support temperature.