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.
Since you are changing the path here, you are going to need to change client/container_info.go since they need to work together. (we should probably have a mini-integration test that validates that this works as expected...)