set the tag's parent frame to the camera optical frame

[christian-rauch]

From the commit message:

The image header carries the 'frame_id' which is set by a camera driver to
the optical image frame. The pose of tags that are detected in an image are
therefore defined in this optical frame.

This makes the launchfile argument and node parameter camera_frame obsolete.

Fixes #100 .

[christian-rauch]

Is this ready for merging?

[wxmerkt]

Yes, it should be - I was waiting to see if someone comments about the removal of camera_frame resulting in an issue for existing users. I don't expect it to be since it should merely simplify usage (the driver should fill this in correctly).