This adds an Exists() interface to detect when the container is dead.
Before reporting an update error we check is Exists() is true.
Some documentation was added as well.
This allows a ContainerHandlerFactory to register a CanHandle() function
which is called to determine whether the factory can handle a particular
container.
This commit disables being able to run cAdvisor without lmctfy. This
should be enabled again with a "no-op" global factory which I would like
to do in a separate PR.
