[PlayerStage-Gazebo] Adjustable Laser scan width

[David C. <da...@gm...>]

I had a need to simulate a SICK scanner that only traverses a 90
degree arc, so I wrote a patch that adds a <scanWidth> attribute to
the SICK model. Default value is 180, so it shouldn't break any past
configs.

This is the first time I've contributed a patch anywhere, so let me
know if I have done something incorrect or inconvenient. Also, is
there a HOWTO contribute a patch or something like that out there? I
was sort of stumbling around the command line diff, and it seems like
it must be easier than it was.

Dave