Move mbf_msgs into it's own git repo

[jspricke]

Hi,

currently mbf_msgs are part of the mbf repo, meaning that they get a new release whenever you do a new mbf release. That's not needed and is irritation cause messages are an external interface.
Please move them to a new repo.

[spuetz]

Hey ;) Are there more reasons for doing that? Because, we have a couple of other issues to solve and I like it to have it all in the same repo. I wouldn't count the versioning as a strong argument. :D Cheers.

[jspricke]

hm.. breaks our internal tooling ;)

[Rayman]

For the maintainers it saves a lot of time keeping everything in one repo

[jspricke]

@Rayman that's not really true, also look at other ROS packages, like common_msgs, control_msgs or moveit_msgs.