Use watch.Filter inside the watch functions

#53 will use the dynamic kubernetes client to watch 3pr resources. The watch events come back as `runtime.Object`s that can't be type-asserted to the proper 3pr types, so we've written a translation function to do so. Whatever code receives on the event channel is required to call that function.

Instead of requiring that func to be called, the watch functions themselves should call `watch.Filter` to return a new `watch.Interface` that internally (via the filter func) calls the translation function, so that none of the receivers will need to.

Requires #53