Add base_frame_id param (defaults to base_link)

The nav_msgs/Odometry message specifies the child_frame_id field,
which was previously not set.
This commit creates a parameter to replace the previously hard-coded
value of the child_frame_id of the published tf frame, and uses it
in the odom message as well.
Merge pull request #94 from pal-robotics/odom-child-frame

Add base_frame_id param (defaults to base_link)