Change ContainerState.java to be Immutable, change startedAt and finishe...

...dAt to be String (as Date does not parse them correctly)

@rculbertson seems like the issue with the Image Id's (in the removeImage test) has come back...

I did tests locally, and Jackson just does not parse the provided dates correctly (it looks like docker provides a higher accuracy than Date supports, possibly Joda date will support this).