Global position plugin

Here's the inital release of the global position plugin. **Needs completion and testing!**
There are some thing that must be added though (not sure how to do it):
* See how to receive `time_boot_ms` and how to convert it to `ros::time()`;
* Probably we have to consider the `NavSatStatus` here too, but I'm not sure.

Please @vooon feel free to give an opinion and some tips on this.

The `gps_conversions.h` file also allow us to convert from local UTM to GPS coordinates, which can also be good on the APM case (considering that this one accepts more than just NMEA convention).

Fix #87.