Use inotify to watch for new containers.

This reduces cAdvisor CPU usage below 1% in my CoreOS system.

We also reduce global housekeeping to every 60s as a fallback in case we
miss an event.
Remove unused filter ContainerHandlers.
Wording changes to SubcontainerEvent comments
Merge pull request #238 from vmarmol/inotify

Use inotify to watch for new containers.