Covariance and Frame_id Fixes

This pull request does two things:

1) Sets covariance to -1.  The ROS convention is to have -1 for an invalid covariance.  This might break robot_pose_ekf because it handles covariance incorrectly.  Setting covariance to a large number means that measurement is valid, but not sure.

2) Allows a configurable frame_id.  The GPS global frame_id needs to be injected in this node, while the child_frame_id and NavSatFix is the frame_id of the antenna, which needs to be a child of base_link.

I'm not 100% sure of your backward compatible use cases, so let me know and I'll make sure this won't break anything for you.  For instance, if you need odom.header.frame_id === odom.child_frame_id === navsatfix.header.frame_id, we could default the 'output_frame_id' to be the empty string, and know that's a special case.