Added launch arg to enable front-facing laser.

Simulator companion to https://github.com/jackal/jackal/pull/9

Usage is:

    roslaunch jackal_gazebo jackal_world.launch front_laser:=true

Concept is that there would be multiple (many?) similar options, eg back_laser, mast_laser, front_stereo_camera, front_camera, etc. We will implement and test these as the payload packages are assembled.

@Shokoofeh, @rgariepy to review.

@pmukherj, if you have a moment, I'd appreciate your thoughts too. Is this a reasonable and scalable approach? I'm assuming a really serious or highly customized user would use these building blocks to create their own description, or at least a wrapper launchfile that passes the relevant args in for them.