Provide Out Of Memory information in container status

Container processes can get killed by the kernel whenever they exceed their memory limit or when the overall memory available in the system is very low. When containers get killed by the kernel, there is no way to figure out if it died due to lack of memory.
To solve this problem, this patch modified docker to Identify out of memory kills inside containers and indicate that as part of the container status string. Since some containers can gracefully handle OOM kills, this patch does not attempt to bring down a container whenever an OOM occurs.

Ping @crosbymichael @vieux 
cc @bgrant0607
Adding SetOOMKilled() would mean that callers will have to do two operations while updating state. And also if we want to add more information to the status, for example 'system memory pressure', we will end up having more and more methods that we will have to call to keep the state in sync. The current change makes the code future proof.