Get root container stats from cAdvisor

I changed the GetContainerStats function, which will return the root container's stats if the container name is empty. This is useful when the master wants to know machine wide information. e.g. total memory usage of a certain node.
The path is the container's path, where the root container is "/", and all docker containers are under "/docker" container. This convention is used in lmctfy, which supports subcontainers. cAdvisor supports both lmctfy and docker, so we adopt this naming convention.

``statsFromContainerPath()`` is a better name and is more idiomatic to Go. I put ``get`` there simply others are prefixing ``get`` in front of all getters.