Add the ability to turn off image pulling.

Clayton: which?  Protocol is an enum (though not enumerated in types.go, probably should be) and is caps.  RestartPolicy is a struct.  Kind is javaStyle.  CauseType is javaStyle.

We have a mish-mash.  I am biased to dislike javaStyle, but I'll run with it if that is the decision.

I nominate @bgrant0607 as decide on api-exposed string constants.  Unless the decision is "inconsistency is OK", we have some work to do to normalize.