Better handling of gazebo_ros run when no roscore started

This checks if roscore is started and gives proper warning message. It also allows the user to easily Ctrl-C gazebo during this while loop and it actually shuts down. This is useful if you are on a single terminal and need to run roscore first.

NOTE: This will not build unless you pull the lastest from Gazebo's default branch, [this patch](https://bitbucket.org/osrf/gazebo/pull-request/595/added-a-signint-event-when-server-is-being/diff) is needed.