setting frame_id as argument in Navigator

[hyaguchi947d]

I met a problem with my robot as follows:

Warning: Invalid argument "/map" passed to canTransform argument source_frame in tf2 frame_ids cannot start with a '/' like: 
         at line 134 in /tmp/binarydeb/ros-kinetic-tf2-0.5.17/src/buffer_core.cpp

It is caused from the frame_id of goal pose, is hard-coded in Navigator.
Additionally, I would like to change frame_id for map having another frame_id.
This pull request will add frame_id option to Navigator,
user can change the frame_id of goal pose.

[Cavalletta98]

When will be merged?