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.
WDYT of instead of introducing this "not active" state from Docker, we try to attach in CanHandle() and if we can't (because it is not active) we say we can't handle it and let the raw driver handle it?