Increase default memory limit to 256Mb
WIP usage command using the cgroups stats files

Conflicts:
	docker_containerizer.py
Switch to default implementation of `wait`
Fully working cgroup metrics, debugging out to stderr
Initial implementation of resource usage
Switch back to `kill` for container destruction (for now)
Removed the -H/--docker-host argument as it no longer makes sense

With the new lxc_metrics methods it's a requirement that the docker instance
you're connecting mesos to is on the same host (for filesystem access) it's
no longer correct to allow the user to specify another docker host.
Full implementation of `usage` command stats
Merge pull request #2 from duedil-ltd/feature/container-usage

Implementation of `usage` statistics from docker cgroups