Update README
Update README

This commit updates the README with project status information and
testing and license sub-sections.

drewkerrigan/mesos-go,basho-labs/mesos-go,mesos/mesos-go,mesosphere/mesos-go,tnachen/mesos-go,tnachen/go-mesos,jdef/mesos-go,ilackarms/mesos-go,chrisjs/mesos-go