Convenience launch file for rqt_bag

[lucasw]

It's useful to have a simple launch wrapper for rqt_bag so that it is automatic to get a roscore running if there isn't one running already - for example getting rqt_bag associated with *.bag file in Ubuntu: http://answers.ros.org/question/256044/open-bags-in-rqt_bag-in-by-clicking-on-one-in-ubuntu/

This a workaround for #238

I notice there are no other launch files in here other than test/test.launch, I couldn't say if adding this one implies every plugin here ought to have one like this.

[dirk-thomas]

I would say it only makes sense to add launch files for plugins which are commonly started "standalone" (without running a roscore). For rqt_bag that is certainly the case so look at recorded bags.

Thank you for the contribution. I have cherry-picked your patch and added the missing install rule in 6d9fdf5.