Recognize parm name world_frame in launch file

[EnricoMingo]

As in the title:
<param name="world_frame" value="desired_frame"/>
should initialize everything with the world frame in desired_frame. This feature was present in the past, I suppose we lost it in some merge.

[alaurenzi]

Please check out the branch bugfix_29_world_frame.
I have decided though to rename the relevant parameter to world_frame_link, because just world_frame was too generic and unclear.

[alaurenzi]

Working