Read command line options from MARATHON_*.

Read command line options from MARATHON_* environment variables like meos-master/slave.

This could be very useful when you start marathon with docker.  If we had a docker container in which marathon was installed and whose `Dockerfile` contains `ENTRYPOINT ["/marathon-0.6.1/bin/start"]`, we can start marathon like this.

```
$ docker run \
    -e "MARATHON_master=zk://localhost:2181/mesos" \ 
    -e "MARATHON_zk=zk://localhost:2181/marathon" \
    marathon
```

However, I'm not a shell script expart, so I would be pleasure to have feedbacks.

Could we also describe which setting takes precedence when both are supplied?