Add fs.NotifyOnOOM for out-of-memory notifications

Docker-DCO-1.1-Signed-off-by: Bernerd Schaefer <bj.schaefer@gmail.com> (github: bernerdschaefer)
Fix nits on chan direction and defer

Closes #9
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #48 from crosbymichael/add-oom-event

Add oom notify event