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.
Aha, I have same question. I just followed the design of ``getContainerIdFromName()``. If you think it's OK to change it to two return values instead of three, I would be glad to do it.