Make the src/mesos go pk 'go getable'

It would help us a lot if the `*.pb.go` were included in the project.
Se we could include `github.com/dominichamon/gozer/src/mesos` as a regular go pkg.

Including the `.pb.go` is "common" in golang, see https://github.com/google/cadvisor/blob/master/container/lmctfy/lmctfy.pb.go for example.
Sorry, I added it back. I removed it because for me the main use was to build the `.pb.go` and now it's not necessary anymore.

Of course, feel free to close the PR if you don't like it.