Change ContainerHandlerFactories to decide what containers they support.

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.
Merge pull request #90 from vmarmol/systemd

Change ContainerHandlerFactories to decide what containers they support.