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.