Added GPSExtendedStatus msg to allow for additional status enums

[agyoungs]

I wanted to add RTK Fix/Float into the GPSStatus message, but #34 already attempted to do this. The drawback is that the md5sum would change and it would potentially be burdensome to all users with previous bag data.

I think a reasonable workaround is the approach I have here. I added a new message that only defines enums. This way anyone can use the extended enums if they wish and they'll be standardized.