Add Event Registry/REST types

This is phrased that way-- see generic.Registry, which makes no mention of etcd.

This function is specifically to make an etcd registry, so of course it has to mention etcd.

If we get a new FrooBobber store, then we'd have a file here with a NewFrooBobberRegistry() function.

Would you prefer that I make a pkg/registry/event/etcd directory for this to go in? I can do that if it makes you happy, it just seemed a little excessive.