Added support for ContainerInfo for mesos 0.19.0

This pull request implements a very simple way of configuring the container image (and options) passed when launching Task Trackers. This can work especially well when coupled with the a [Docker Containerizer](http://github.com/mesosphere/deimos) as a way of managing system dependencies requires for running Hadoop jobs.
Fixed a NullPointerException that caused no tasks to be launched
Fixed NullPointerException further down
Updated the README to reflect container changes
Fixed wording, err...
Added an extra section to the readme about opt-in
Merge pull request #20 from tarnfeld/feature/containers

Added support for ContainerInfo for mesos 0.19.0