Add proper covariances to NavSatFix

- Calculated GPS covariances based on http://answers.ros.org/question/10310/calculate-navsatfix-covariance/?answer=15197#post-id-15197
- Use lat_fused and long_fused for less drifty values
hdop is the horizontal [dilution of precision](http://google.comhttp://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)).