Bug fixes and introduces network stats.

Added Network Stats to Container Info. It still not exposed via the HTTP UI.
Bug fixes:
1. Modified docker handler to use libcontainer.GetStats instead of quering the fs package.
2. cAdvisor will not stall if any of its core operations fail.
3. cAdvisor will safely ignore any inactive or leaked docker containers. When containers are leaked cgroup state exists but docker is not aware of them.
I am not able to detect the exec driver used for a container as of now. We can deal with that in a separate PR. Added a TODO for now