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)
Replace pid and started file with State type
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Update readme with state.json changes
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Update code based on review comments
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Rename to InitPid
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
Merge pull request #50 from crosbymichael/create-state

Create state
Improve nsinit usage instructions

nsinit must be run as root.

Tidy up the README somewhat to clarify the distinction between
libcontainer and the nsinit CLI.

Fix some typos in other files.

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)