Create blort_ros_msgs + Allow rosbuild/catkin switch

Hello,

This PR addresses two things:
- Create a separate package to hold the messages and services related to blort_ros, this is particularly helpful to make use of BLORT without having to compile it
- Allow to switch between rosbuild and catkin: it defaults to catkin

I have not been able to test the compilation using catkin so I'm interested to know if it's working.

Best