upgrade to 0.20 to get docker support
Merge pull request #18 from kienpham2000/mesos-0.20

upgrade to 0.20 to get docker support
update ami image to ubuntu 14.04 LTS, amd64, ebs
update pre-requisites
Use marathon-0.7.0-RC2 for mesos 0.20.0.  Now user can deploy docker containers in Mesos+Marathon cluster!!
Update README.md
support chronos for multinodes