* Add meta pkg. Move ar_track_alvar_msgs package.

* [msgs] Adjust version so that we can release the package at once within the metapkg.

Currently, ar_track_alvar consists of 2 packages, `ar_track_alvar` and `ar_track_alvar_msgs`. These are maintained in two separate repositories, which duplicates release tasks. These two packages can easily be maintained in a single repo and maintenance can be reduced. Backward compatibility should be perfectly retained since we're not changing anything in 2 existing packages.

To list the actual change:
- Add a metapkg `ar_track_alvar_metapkg`
- Move ar_track_alvar_msgs` from [the separate repo](https://github.com/sniekum/ar_track_alvar_msgs) into this repo.

* [CI] Add ROS Lunar, and allow them to fail since the dependency might not be fulfilled yet before its official release.

* Update readme and .rosinstall files.