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.
This should be a member of dockerFactory. We should refactor Register() to make that possible. Not at all for this PR though :) maybe add a TODO so we don't forget it.