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.
Look below. fileNotFound is treated as a special error. I could do string matching, but I don't like that approach. This error is being consumed by its caller.