Change ContainerState.java to be Immutable

This object is a docker response object from a call to docker, in no point during the life of docker-client should this object dat be changed, except by querying docker again for state (and getting a new object).
Therefore there is no need to have any setter methods on the objects.

For tests I added json fixtures that are the way the object is being created in runtime anyway - a much better way to test than using set methods.

For more reasons why immutable is better (where relevant as in this case)
http://www.javapractices.com/topic/TopicAction.do?Id=29